A large amount of the things you mention become less dangerous in the event of greater alicorn presence in Equestria, not more. Nightmare Moon, Discord and Chrysalis ALL almost won, and if even just a few dozen alicorns had existed, they wouldn't have stood a chance in hell.
Now, the whole existential risk angle...is a very interesting point, since based on what I've just argued, the logical meeting-ground between the two would be to have a task force of alicorns, say, at least a dozen, but no more than a hundred, all comprised of ponies Celestia trusted sufficiently. The chance of alicorn-related existential risk increases, but the chance of the next season's villain killing everyone plummets to nearly zero. So, given your predictions on the power of alicorns, you're right. If we take the prior that alicorns automatically gain Celestia-level powers, it's far, far too dangerous to give everyone that kind of power, and immortalising everyone is a very, very bad idea.
In fact, this very argument leads me to believe that, in order to provide the optimum amount of conflict in the story, alicorns need to be a lot more powerful than unicorns, but not automatically god-tier. Alicorns should have the potential to reach the power of Celestia and Luna, but imagine if Celestia and Luna were immortal unicorns: Based on their great amount of knowledge, they would still likely be more powerful mages than any other unicorn alive. So this could easily extend to alicorns as well. This still brings about the existential risk angle. Powerful mortals can cast spells like Want-It-Need-It and the altering of Parasprites already, but a lot more ponies would be capable of such things if they were alicornified. My own personal belief about alicorn power levels subscribes to this idea, but as another LWer pointed out, I shouldn't make the world convenient for me. I should make it as inconvenient as possible while still allowing the protagonists to win, because that makes for a much, much better story than "Deathists are always wrong about everything forever." But I don't think this is a problem that allows rational protagonists to win. They'd have to back down.
As for your FAI question: The answer is, no, I don't want to convince a Friendly AI of this, but Celestia is not a friendly AI. She's immortal, she's the ruler of Equestria, and she's definitely much wiser than just about any mortal, but she's not a superintelligence. She's not so far beyond ponies in mental ability that the concept of challenging her judgement is ludicrous. She has pony-level intelligence, just a lot more years to learn things. But, as we can extrapolate from elderly humans, sometimes age has it's deficits as well, making people more inflexible in their opinions. Your argument for existential risk is what would convince me if I were Twilight, not Celestia saying it's too dangerous and me blindly trusting said judgement. Celestia knows more than Twilight, but not so much more that in an argument between the two, Celestia can never be wrong.
In fact, this very argument leads me to believe that, in order to provide the optimum amount of conflict in the story, alicorns need to be a lot more powerful than unicorns, but not automatically god-tier.
Wouldn't that weaken the already-weak pro-death argument?
EDIT: I am now taking arguments for alicornism. Alicornism being the placeholder term I've given to the stance that all ponies should be alicorns. Please PM me or post here if you have a good one, or an argument against one of anti-alicornism's strongest points: Overpopulation/over-use of resources, magical abuse/existential risk, or upheaval of the respect ponies have for their rulers due to their alicorn status. I would prefer general arguments for alicornism over counter-arguments if possible. Deathist / anti-alicornist arguments are still fine to post here.
Disclaimer: I'm not sure if this is worthy of a discussion post, but I figured, given the amount of people on LW who like My Little Pony, it would have at least as many potentially interested people as a regional meet-up thread would, so I figured I'd give it a shot. If this is too trivial or frivolous for LW, feel free to tell me and/or downvote, and I'll refrain from such threads in future. A place where I could go to find some help instead of the Discussion section would also be greatly appreciated in such a case.
So I had an idea for a one-shot or small novella, depending on how the plot developed, about an argument between Twilight and Celestia. Twilight finds out she's immortal now that she's an alicorn, and Twilight then decides that, given the standard anti-death concepts that immortality is good, death is bad, and so on, they should turn everyone who wants to be an alicorn into one.
The problem is, I'm having a very difficult time coming up with actual arguments for Celestia.
- Celestia herself is immortal, she's lived for well over a thousand years, and she isn't horrifically depressed, so clearly, immortal life is worth living and there's enough stuff to do with an extended lifespan.
- For the purposes of this fic, it's possible to turn anypony into an alicorn. I'm likely going to go with the idea that the spell can only be used a few times a year, but that's still enough to turn anyone who wants it into an alicorn within a couple of decades via exponentiation: The first targets can all be gifted unicorns who can be easily trained to use the magic.
- In most of the "Immortality sucks" fics I've read, the only real argument that immortality sucks is that you have to watch everyone else grow up and die. If a large majority of the population were turned alicorn, this wouldn't be a problem anymore.
- Nothing in canon suggests that there's any sort of religion in Equestria. Even in fanfics I've read, I've only read one fanfic where someone made up an afterlife that some ponies believed in, and in many more that I've read, Celestia's name is actually used in place of God in various sentences, like "Oh for Celestia's sake!" Thus, it's unlikely they'd believe in an afterlife: Both in canon and the majority of fanon, the closest thing to a God appears to be Celestia herself.
I've come up with arguments for Celestia by roleplaying the argument out by myself, but I haven't come up with anything that Twilight can't just shoot down, and I'd prefer if the argument wasn't just Celestia getting steamrolled, and I'd like to do this by strengthening Celestia's side, not weakening Twilight's.
Is the argument for deathism really that weak? I've read over the Harry vs. Dumbledore deathism argument in HPMOR several times looking for ideas, and IIRC Eliezer actually claimed he steel-manned Dumbledore's position, but I don't find anything Dumbledore says convincing in the slightest, and ended that chapter feeling that Harry was the clear winner in that debate, and that's with Dumbledore having access to arguments that Celestia doesn't, given that in the Potterverse, nobody actually knows what it's like to be immortal, and Dumbledore believes in an afterlife.
Some other arguments I've come up with for Celestia:
Argument: We can't just have a massive ruling class.
Response: There's no need for alicorns to be royalty. "Princess = Alicorn, Alicorn = Princess" is only something that law and tradition dictate: They can be changed. After all, Blueblood is a prince and not an alicorn, and it's certainly possible for an alicorn to NOT be royalty, if the princesses wanted.
Argument: Harder to keep the populace in line, if everyone has more power.
Response: Celestia's not exactly going around fighting criminals herself with her alicorn powers, so Celestia being much more powerful than others isn't necessary to keep the peace. If anything, an alicornified populace is MORE likely to be able to govern itself: Atm, a pegasus criminal can only be pursued effectively by about one-third of police officers, for example.
Argument: Overpopulation.
Response: One response to this is the idea that, starting a year or so from a royal edict, ponies who wish to be changed into alicorns aren't permitted to give birth more than once or twice. A broader response is that "overpopulation" isn't actually a reason to oppose alicornification, it's just a problem that has to be solved in order to do it. Saying "There'd be overpopulation" and then forgetting about the entire idea would be like Twilight saying that they didn't know how she was supposed to save the Crystal Empire from being banished again when she got given the task, and responding to this by saying "Oh well, guess that's it, we may as well pack up and go home." rather than trying to actually solve the problem. That said, this is the only truly legitimate argument I've come up with, an argument that requires real thought to fully defeat, rather than an argument that has an easy response leap to my mind.
Argument: Mortals wouldn't understand the consequence of their decision.
Response: Again, several arguments for this. Firstly, there's no reason to believe the alicorn transformation is irreversible, even if it's not currently known how to transform it back. Secondly, Celestia can already predict the consequences, and since she thinks HER life is worth living, clearly there's a solid chance that other ponies will have their lives worth living as well.
So, the questions to ask:
Are there good arguments for Celestia I haven't thought of?
Are the arguments I've already posited sufficient to not straw-man the lifeism position, and to allow for a reasonable argument?
EDIT: I am now taking arguments for alicornism. Alicornism being the placeholder term I've given to the stance that all ponies should be alicorns. Please PM me or post here if you have a good one, or an argument against one of anti-alicornism's strongest points: Overpopulation/over-use of resources, magical abuse/existential risk, or upheaval of the respect ponies have for their rulers due to their alicorn status. I would prefer general arguments for alicornism over counter-arguments if possible. Deathist / anti-alicornist arguments are still fine to post here.