I wanted to make the narrative support deathism to the point where it didn't seem like an obviously false position to the reader. I tried to do this by making the deathist advocates the characters who the fandom associates with reason and wisdom, by making the anti-deathist advocate aggressive and uncompromising, and by emphasizing that most everyone liked the status quo just fine. I stopped doing this sort of thing in the last act. (In hindsight, "steelman" is the wrong term for what I did. Sorry about that.)

I think I was successful at the above. Readers who didn't start as transhumanists were unable to tell what my own position was.

Celestia is smart enough to use every dark art known in order to convince others

Smart enough, but not vicious enough. This characterization of Celestia wouldn't do anything she felt was dishonest. My intent was to show her as a tragic character doing the wrong thing for understandable reasons.

I'm still not sure why the new alicorns put up with Celestia's rules; once she resorted to threats of violence, it seems like it should have been pointed out she would lose a contest of violence.

Partly because they are happy colorful ponies and they prefer not to solve problems through force. Partly because Twilight still craves Celestia's approval.

Thanks for all your feedback; this is a useful conversation. I am strongly considering taking your earlier suggestion and adding the "it's just ponies" theme to the big confrontation with Celestia.