Bacon Bits
Legend
I'd say I'm fine with the changes. I think big change was 2E to 3E, and since then it's been more of a drift than any major changes in art style. 2E still had that old-school look, even quite late on, but also some artists taking risks and being, well expensive-looking artists like DiTerlizzi and Brom. Whereas 3E went for a more straightforward "illustration-y" style, which was definitely more modern, but was less artistically interesting.
My problem with the last twenty years of D&D art-wise has never really been the aesthetic in a broad sense. It's been that most of the art just hasn't been very good technically, nor original, nor even particularly evocative, and I know some people are going to be mad, and WAR defenders will be particularly mad to hear me say that. But may WAR should have been less boring? I dunno. He certainly set the tone for like 20 years of D&D.
That didn't change with 4E, and it hasn't changed with 5E, except where 5E has employed MtG art, which generally isn't like a cut above D&D art, it's like a Zorro Z above D&D art in terms of how evocative it is, how strong some of the pieces are, and so on.
Currently D&D is drifting into a particularly generic "modern fantasy" style that exists solely because there is so much fantasy now, and illustrators have sort of converged on this particular approach, which I've described before. I don't hate it, but it is boring. It's less irritating than some approaches for sure, and closer to the "imagined aesthetic" of most D&D sessions I've been involved in than some styles.
I don't think anything you've said here is wrong, but I think 2e fell into that same trap of generic fantasy. I remember Elmore's works generally looking like the front cover of every fantasy novel published at the time. It was, perhaps as a victim of his own success, the generic image of fantasy that I recall from Dragonlance on. I think that's why they went with Brom and DiTerlizzi: to change things up with new looks. They specifically went with a single artist for a whole setting. Planescape and Dark Sun have consistent art direction because they had a single artist. However, TSR books in these settings also have a lot of pages that are just two columns of text.
I don't think WotC has done any book with a single artist, let alone every book in a given setting. Maybe not even done concept artwork. WotC's books have been larger, and with more art than TSR books. It's just not possible for a single artist to produce enough pieces for a modern WotC book. For real, grab the 5e PHB and flip through the race and class sections. More pages have artwork on them than don't. In some sections there is artwork on every page. It doesn't have the quality that Planescape and Dark Sun books had, but.... there's a lot of paint there, where it would be a line drawing in 1e or 2e. I agree the art has changed and isn't as consistent or evocative, but I think that's at least partially because audiences demand so much of it. I can't imagine that WotC's product leads can spend as much time on art direction as TSR's books did.
Never mind how much cheaper 5e books are compared to their 1e counterparts (after adjusting for inflation). $20 in 1978 is nearly $75 in 2014. That difference has to come from somewhere. Would you happily buy an $80 TTRPG book today if it wasn't in full color? I'm not sure many people would.