I just realized that it's not actually known whether the Ethereals would actually destroy humanity.

How sure are are we that the Ethereals were out to melt or modify all of us?

It could very well be that we keep living under our glorious overlords while a few thousand a day are "kidnapped by terrorists" and sent to conquer other planets as brainwashed Spess Mehreen units that lead pods of Mutons or whatever.

In fact, the ritual scarring on Mutons and the decorative patterns on Fuckyou Armored Personality Cores suggests that there are enslaved populations of Krogans and Tentacle Monsters out there that still practice their original cultural rituals. Note also that the Ethereals were harvesting humans, not cultivating them.