I don't think this analogy works at all because the "LotR" and "Silmarillion" of Greyhawk already exists in that there is a ton of material out already. I think a more accurate analogy would be, after the Dragonlance Chronicles and Legends were published, asking "why more?" The More became an endless stream of novels and short stories, and then later books revisiting the characters from the original series...and the whole thing got over-saturated and, quite frankly, beaten to death.
We see this time and time again. Rather than a new science fiction franchise, we get a re-hashing of Star Trek, but this time for Gen Texters. A lot of the original vitality is lost - especially when you try to re-create things like the chemistry between Kirk, Spock, and Bones, or the classic villainy of Khan. I would have rather seem them start with an Enterprise after the Next Generation - with a clean(er) slate. The death of Kirk scene in the recent Star Trek movie ended up being almost comical in comparison to the epic drama of Wrath of Khan.
That's all nice in theory, but there's a big problem: those names
sell. That's why we keep seeing sequels, prequels, reboots, and rehashes (plus star vehicles for the likes of Pitt, Cruise, Tarantino, Spielberg...) That's why Disney just bought Lucasfilm, why we're getting a new trilogy of "Star Wars" films, and why we'll all go trooping off to see them in the cinema, even after the epic disappointment that was the prequel trilogy.
When it comes to settings, WotC
could try out a new setting, and they
might manage to catch lightning in a bottle and hit on something that becomes as big as the Forgotten Realms. But, much more likely, they'd get something as big as Spelljammer, or Birthright, or... something that does well enough for a while, but isn't really a huge seller long-term.
Or, they can put out a new version of FR, and be guaranteed big sales. Probably not as big as last time, and it probably won't be as well received as last time (well, okay... the time before last), but probably better than "the next Birthright" would do.
And given that it costs the same to do the one as to do the other, they choose the safe option.
(And, of course, that's why 4e was "Dungeons & Dragons", and not just some new fantasy RPG, and why 5e is going to be "Dungeons & Dragons". And 3e, for that matter.)