What a ride it was! Thank you for your hard work, really! Still thinking that mechanically a good errata would have fixed the original books stat blocks, maybe telling for every spellcasting monster a "signature spell" that is the primary attack source for easier running on the table, or directly the magic attack. This without making a new book.
The MotM operation was done to compile the monsters stat blocks in the "Core Extension Set" (3 books instead of four), and to completely "delete" the old lore chapters from VGtM and MToF: from WotC standpoint, they need to be all rewritten, and just think the backlash for just the lore errata last year: the more they errata, the more the community will grow angry. Better start on a blank slate, and when time will come they will release new books/DNDBeyond exclusives with sanitized lore.
1: When they want more monsters. Normally around the time they have done a campaign or two and are ready to start fully creating their own adventures.
2: The fact there is a canonical dwarf who wields an actual flamethrower. That was before Spelljammer was confirmed and anything like that was absolutely wild at the time.
3: Well, aside from the things I listed in my conclusion, I would have wished for a more robust monster equipment ruleset. I know I'm not the only one who wants their PC to get their hands on that flamethrower, and quite a few other monster exclusive weapons would be cool to have too. More importantly, I want that Rock attack in the hands of more monsters, it's just so good for ranged coverage.
4: The Kobold Inventor, but it wasn't drastically changed. If you are asking my favorite changes, it would go to the Warlocks.