Gershwin's "Rhapsody in Blue" was seen as watered down and full of weak melodies influenced by the 'lowbrow' jazz that you mentioned, now it's a national treasure.

Maybe the premise is muddy because I have no horse in the race. Alot of critics to me chase aesthetic and trends as much as fans do, except they like 'outsider' aesthetics moreso. That said I argue that there was more over-lap some time ago, and even if the critics resisted new musical styles initially, they still accepted after a while. Then I think that pattern broke down, where increasingly top 40 music was derided by critics, till now, where the two rarely meet. I think critical darling Bon Iver's uncomfortable grammy speech is emblematic of this.

I believe I led you astray with the last sentence in my opening post, the 'opening shot in a civil war' thing, making it seem like the rift wasn't always there when it was. But
i feel like both camps drifted further apart at some point, I just don't know when.