I think what you might be missing is that "they" has been a different group of people each time since the original two games—this is the first Ron Gilbert Monkey Island game since LeChuck's Revenge. I believe Gilbert has also said that he's disregarding everything in Monkey Island "lore" that he wasn't around for the writing of. because of all of this, the franchise has a general identity problem, including a visual identity problem. most people who are excited to play Return to Monkey Island are not coming from a place of "well most of the rest of the games looked different from one another, let's see something new!" I think most people expected the franchise to either maintain its new "Remastered" look, or look closer to the original two games. personally I'm glad they chose neither!
the backlash only seems disproportionate due to reporting on it. "New Game In Franchise Looks Different From Old Games In Franchise, Many Anonymous Internet Users Vocally Upset About This, Like Everything Else, As Per Usual" doesn't really have the same emotional galvanization effect, see?
but again I agree that the new style is actually a neat compromise between the old cartoony pixel art character sprites and something totally different, without going down the full-cartoon-whismy route that the later titles and Remastered games took. part of the charm of the first game is how expressive the characters without being too over-the-top cartoony like DoTT or Sam & Max. the new style looks like it should be able to recapture this feeling quite well!
again, the complaints are stupid, but should have been very easy to expect, because pleasing everyone—especially when it comes to nostalgia—is basically impossible.
But why? They've done this exact thing N times before, this is just the N+1 iteration of it.
You also keep positing alternatives they could have done and claiming that those alternatives would have been better. But it's all based off of your conjecture. You claim it's self-evident, but it's only self-evident in the way that is evident to yourself.
The backlash seems out of proportion to what was done.