Question: why are regular bats etc completely replaced by elites?I think it's because bats are boring by the time you have killed X of them. Think of the elite versions of all monsters what they should have been all along, only they were holding back to be nice
Exactly. Also, the point is to never visually distinguish the elites: they are new behavior modifiers for existing enemies, not entirely new enemy types.
Regular bats are okay when you're learning, but past a certain point they just aren't interesting at all. No amount of new enemies added into addition to bats will keep the bats from diluting things and making it so that there are always pockets of boring along with the pockets of awesome. Hence the two-lane approach: elites that replace, and brand-new enemies that augment.
Very welcome to feedback on thresholds for adding of elites, though; I had wondered if perhaps 80 wasn't way too low. Then again, you fight a pretty good mix of enemies (which will continue to be expanding), so killing 80 bats isn't like killing 80 enemies in general -- it might be killing 1600 enemies, if there was a bat you killed for every 19 other enemies you killed (and that seems high, really).
Given how people tend to progress at the game, killing 1600 enemies seems like it would take a really long time, meaning that the elites really wouldn't be showing up for a while for most enemy types.