Sometimes history moves slowly. During his life, Bach was best known as an organist; sure, later composers studied and loved his work, but it wasn't until the mid 19th century that he started to get the reputation that he has now.

I thought someone would mention that. I think it's different. Schoenberg et al. were famous while they were alive. Their works were performed publicly, and adored by the cogniscenti, for decades. Bach grew into public favor. Schoenberg fell out of public favor. He had every chance the music establishment could give him, and still fell out of favor.

(BTW, Haydn, Mozart, and Beethoven all made special studies of Bach's music in the 18th century; so I'm skeptical of the "Bach had no reputation as a composer" argument.)

Also note that the time between when Bach wrote the St. Matthew Passion in 1727, and when Mendelssohn "revived" it in 1829, was only 102 years. We've already had 100 years of Schoenberg.

Also note that Bach is always brought up in this context because he is such a notable exception in that way

However, I think there's another factor at play here - "art music" experienced the same academization and post-modernization that we saw in the visual arts. Serialism, musicque concrete, aleatory composition - all these things pushed the boundaries of what "music" actually meant, going against popular sensibilities in ways that (and I could be wrong here) the "art music" of previous centuries did not.

I agree completely.