D&D NEEDS a bog-standard, no-frills setting that highlights the rules and lore of the core game. Forgotten Realms really does fit that bill.
I hate to bring it up again, but take Golarion. Its a bog-standard world; complete with giant Metropolis, a fallen empire, evil nation controlled by devils, Transylvania, fantasy Egypt, proxies for Asia, Africa, and the Middle East, all surrounded by a poor-mans Great Wheel. And its All Pathfinder Has. Every AP deals with it. Every story happens there. Every sourcebook details it. All artwork references it. Pathfinder CAN be played without Golarion, but as far as Paizo is concerned, Golarion = Pathfinder.
And I rarely see any PF players complain about that. It boggles my mind.
The Forgotten Realms, in 5th edition at least, is D&D's Golarion. Its bog-standard, no frills world that has proxies for most cultures and settings, highlights everything in the core books, and is big enough to absorb (or at least tolerate) the best of D&D lore, even if its imported from other worlds. Heck, 5e has a lighter hand with FR than PF has with Golarion; at least 5e makes token references to thing beyond Faerun. Yet for some reason, the idea of a bog-standard world makes some people retch when it comes to D&D.
Why aren't the PF boards full of people demanding new settings? Why aren't there "Why Golarion is hated?" threads? Seriously, why aren't people on Paizo's boards demanding APs that don't deal with Golarion, or refusing to buy books that say "X of Golarion" on them?
D&D : Forgotten Realms :: Pathfinder : Golarion. Its the default setting for marketing and branding, the catch-all for whatever new stuff comes down the pipe, and the minimum standard for people too lazy or uninterested to use something else. That, IMHO, is a strength for both games.