Soooo... you've basically admitted that you have no counterargument? Given that a response like "shut up" when presented with an argument is always invalid. Argument here meaning: a series of statements typically used to persuade someone of something or to present reasons for accepting a conclusion. And not the more common meaning. Although not to say the former can't lead to the latter but I digress.
In that case I wonder what type of cultures and main races of monsters would arise given enough time.
That's because you're basing it on your own imagination without solid fact. For one fact, Sin hasn't even appeared in the story, if at all (just some guy decided to say so. Dunno if it's true or not)
And it's highly doubtful most monsters would evolve passed a fiend. Unless they were genuinely born as a fiend. Fiends created by humans, are basically just resentful, revenge driven creatures/humans. So, I'd imagine a more apocolypse setting rather than any 'monster society', your thinking up.
That and your trying to make dying to be a good thing.
Which it isn't.
Sure, they can go to a restful heaven, but doesn't mean they didn't want to die early. Out of billions of people, pretty sure as human nature, they don't want to die. Some might. But most turn into fiends or unsent because they don't.
And there are also the unsent who still have a human form. And can still fight apparently. So, it's also possible 'human' wouldn't be extinct even if Sin wiped them out. They'd just be physical ghosts. Most likely rare compared to the amount of fiends since unsent aren't vengeful. Human's turned into fiends, those fiends 'evolving' would most likely just turn into unsent.
And, you're being told to stop because you just joked about abuse and sexist, you got told to stop, then switched over to some random ideology as some other joke as it sounds.
It might be amusing if it didn't come off the first joke. But now, everyone is just mildly annoyed.