Oh, hey, most of the stuff you see written now has at least been influenced by what came before. Of course. My point is that you simply don't have the sheer volume then as what you have now.
According to Locus Magazine, in 2000, there were about 200 fantasy novels published. In 2001, there were almost 300. There weren't 300 fantasy novels written in entire decades before 1980.
Yes, we should keep some of the older stuff. Hell, I like some of the older stuff, but, dismissing newer fiction is a serious mistake.
One thing I find very curious actually is this idea that somehow D&D was based on small sections of fantasy previously. That's just false. D&D has ALWAYS been the kitchen sink game. Whether it's Gygax throwing WWII nazis at his D&D players, or Lost World pastiches like Isle of Dread (and no, The Lost World is not fantasy), gothic horror like Ravenloft, SF elements in Barrier Peaks and high fantasy in Dragonlance, classic children's lit in Dungeonland and Beyond the Magic Mirror; on and on you've had D&D incorporating pretty much anything and everything.
That's D&D's greatest strength. That you can use the system to borrow from pretty much every fantasy tradition. However, that comes at a cost. D&D doesn't emulate genre very well. You can do high fantasy for example, but, D&D is too lethal to do it well - thus we have Raise Dead spells. You can do traditional Sword and Sorcery, but, D&D is too high magic to do it well - too many races, too many spell casters. You can do steampunk, but, not particularly well. On and on.
D&D does D&D very, very well. And what D&D is, IMHO, is a fantasy that isn't really any single genre. It's Fantasy (big F) with all sorts of fantastic elements jammed together to allow people to have a blast killing things and taking their stuff.
I can't imagine D&D designers suddenly closing doors and ignoring wide swaths of fantasy out there. They never have in the past, why should they start doing so now?