Faulkner, Thomas Bernhard, John Barth, Henry James, Herman Melville, Fleur Jaeggy, Dostoyevsky, Marguerite Duras, Poe, Hawthorne, Rosemarie Waldrop, Kraznahokai,

These are just a couple that came to mind. Among them, probably Waldrop, Jaeggy, and Bernhard are the most experimental, but I would argue that none of them aesthetically speaking write books that are simple, and I don't think I could argue that any of them should have simplified their themes or style or general employment of language to be more accessible.

Kraznahorkai and Bernhard are great examples. Are walls of text without paragraph breaks harder to read? Yes. But this is an important aesthetic choice. In both cases (all of bernard, melancholy of resistance for Kraz) it speaks to an overbearing oppressiveness that ties directly into their thematics. If you missed this I think you missed out an essential point of their aesthetic and what they were trying to say. We cannot sever form and content. This is why I think it's absurd to complain that someone's work is "not accessible" —its really silly to demand any sort of aesthetic capitulation on the part of any artist, literary or otherwise, in the first place.

Edit: Faulkner is another good example that's less experimental. I'm sure some readers would have found As I lay Dying or The Sound and the Fury more accessible if a narrator mediated between the various first person voices he presents, but this would so drastically change the aesthetic character of these works that I doubt you'd be able to claim they aren't essentially different and would not be equivalent pieces of art.