You say "objectively," but unless you confirm that's what you meant, I'll assume you meant "with sufficient evidence to justify," because I think the latter is what you meant.
There is some uncertainty as to when happiness would end and pain would begin (we're dealing with a fictional universe with unknown (and probably undecided) physical laws, for Adun's sake), but the wikipedia article on the big rip says that if it is indeed the fate of the universe, space will stretch infinitely much in a finite amount of time. If that's the case, then it ind of makes no sense for an alicorn to survive. It's like an unstoppable force and an immovable object. But if alicorns are alive at the moments leading up to that point, the stretching on them will grow to whatever level it needs to (and quickly) for it to be torturous. And if they are somehow alive after that point (not sure what that would mean even) "The torture gets less then" sounds like the least sensible option.
So maybe you can't say exactly when every alicorn is getting tortured, but given that the big rip happens, every possibility is bad. And even if a few alicorns weren't tortured (while most were), unless back in Equestria billions of years earlier you could foresee which ones they were gonna be and only make those ones immortal, it's better to kill everyone (or everyone except Luna, Celestia, Cadence, and Twilight, who are fortunately all female and perhaps unable to continue ponykind (unless magic works like it does in a certain fanfic I read...)) than to let that happen.
I meant objectively, unless you are limiting your response to whether you personally would create alicorns; I asked if it should be permissible for alicorns to be made.
Suppose the physical laws are not well enough determined to be sure if the big rip happens, or if there is promising developments in magic that have some chance of preventing it, or some other reason why it is unsure if the happy fun time will be finite or not. How large a chance of "finite good time followed by eternity of torture" is acceptable, if the remainder of the probability space is "infinite good time"? I think multiplication does no good here.
I recently published Mortal, a novella-length My Little Pony fanfiction meant to introduce anti-death concepts to an unfamiliar audience. Short description:
This is a character-driven melodrama. It's not particularly rationalist, but it's very, very transhumanist. Unlike, say, Friendship is Optimal, I wouldn't necessarily recommend this one to people who don't already know the source. It assumes familiarity with the characters and the world.
I am going to talk about how I put together the story and how people reacted to it. This will contain spoilers.
This line exists so you can break out of the automatic "read everything on the page" mode if you want to avoid the spoilers.
This story was structured as something of a bait-and-switch. I watched the reaction to a previous transhumanist horsefic (yes, there's more than one), and I was struck by how easily readers matched the explicitly anti-death narrative to the "immortality is a curse" trope. Rather than fight against this trend, I decided to work with it. The first act is meant to look like a story about learning to accept the inevitability of death. Starting in chapter 3, I break further and further away from that mold until the protagonists finally rebel against the status quo.
The first chapters got a lot of people invested who I suspect would've been turned off by a less familiar opening. Once I was into the third act, I stopped being subtle and used every trick in the book to make the pro-death characters look like the unreasonable ones. Judging by the comments, there's no shortage of readers who were angry at having their expectations flouted, but quite a few seem thoughtful, and some explicitly changed their mind on the subject.