I don't think this is the right place for fiction advice, but I'm upvoting anyway because your question is interesting independently of the fiction. That said..
On the one hand, you shouldn't expect to find a very convincing argument for deathism. Given that it's the status quo, if there were good arguments for it you would probably be a deathist!
On the other hand, there are arguments specifically against being turned into an alicorn which would also apply to most realistic real-world ways to prevent death.
Specifically, you get turned into an alicorn, which is not the same race you've been the rest of your life. If you're a pegasus, you gain an additional sense for magic and probably lose a bit of maneuverability, if you're a unicorn you gain wings, and I hardly need to explain why the poor earth ponies might object.
That, alone, is probably enough for many people to postpone the process for a while.
Other possible arguments...
It would reduce diversity. Well, it would, though that seems rather selfish.
Alicorns are scarily powerful, being able to move celestial objects around and all. Some analogies to nanotech-powered antiagatics here... Equestria might end up destroyed in short order, or you'd need much stronger internal monitoring to prevent that. It's easy to come up with downsides along these lines.
It might easily cause the greatest economic recession in history, as it attempts to adjust to vastly altered outlooks and changed capabilities. In particular, if earth ponies lose their 'talents' they might lose a lot of their industrial base; controlled magic can probably replicate it, but not everyone will have the intelligence for that, and in any case it'll take a while. Still worth it, but it'll be a hard few years. Or decades. Eventually people will realize they have decades.
That's...an interesting point. I never actually thought that Celestia and Luna could move the celestial bodies because they were alicorns. I always just thought they could move them because it was their special talents, and it was unique magic they could do because of their knowledge or talent, not because only they had the brute force to do it. After all, the only fight Celestia was ever in canonically, she lost, and it wasn't even all that climactic either.
In the event that all alicorns have royal-sister levels of power (Again, my assumptions have blinde...
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.