>>152756Both the good and evil isekai groups still ended up help helping humanity grow from said groups butting heads with all the stronger races and species that would've easily enslaved or wiped them out, until Ainz who doesn't have much going against him to distract from world domination besides the Holy Kingdom and the dragons potential strike team.
Kazuma's predecessor, while having been one of the people chosen to be a hero, has been responsible for almost every single major threat that has happened in the series; giant mech that goes around obliterating towns and killing anyone in its path that even the demon king avoids, race of chuuni that eventually gives birth to a girl who drives one of the demon kings generals to the point of wanting to destroy a town, creating a weapon that made of the generals practically unbeatable, and if you count Isekai Quartet, and got isekai'd again and created a giant golem whose only way to shut it off was using an emergency shutoff that woudln't have been usable had it jumped worlds.
Hitler on the other hand helped humanity come together and grow strong enough to subjugate the other races but this eventually leads to the Elves, Dwarves, and all other races having slowly deteriorated towards extinction, so when Hitler's gone and a new group of isekai'd people show up and gather the monsters to wipe out all the humanoid races; it could be said they'd probably have had some kind of a chance without isekai of their own, if they had came together and strengthened each other up instead of trying to wipe each other out beforehand.
Ainz has self control and has the desire of humanity willingly giving itself up to him and not enslaving them, all the predecessors leftover shit are all mindless engines of destruction that have the capability of wiping out humanity if they're all dumb enough to try to come together and fight them, Black King just wants humanity dead for some reason and will kill or subjugate anything in his way which is pretty much what I'd think it would've panned out for Ainz had he not cared about keeping humanity around.