This would be more convincing if Forgotten Realms was enormously popular as opposed to being a setting that only interests a small minority of D&D players (a larger minority than other D&D settings, but still only a minority).
It was obviously a while ago, and so things might have changed, but it certainly used to be the case that the overwhelming majority of D&D players used no published setting at all (and, of course, very few ever used more than one). That's why, for example, the 3e FR books were sold at a higher price-point than the equivalent book would have been had they 'just' been labelled as D&D books - there was a price premium for the FR sub-brand.
Because of that, I've found 5e's focus on the Realms somewhat odd. (Sure, it's just one of several settings mentioned in the core, but it's also the 'native' home of all the published adventures so far, and now of the one and only non-adventure supplement.) That would seem to be a way to unnecessarily lose a bunch of sales from people who see the FR-ness and immediately check out.
It's an odd strategy... if the focus of the strategy is on maximising RPG book sales.
But that's the thing: it's pretty clear that that's
not the focus - WotC have been pretty clear about their cross-media strategy. And one of the problems that gets raised in every discussion of the D&D movie is that D&D doesn't have a natural 'story' of its own. Well, now they have three: Tyranny of Dragons, Elemental Evil, and Rage of Demons.
I strongly suspect that it will be easier for them to sell a D&D movie (or TV series, or whatever) to people who don't know what it is if D&D = FR and vice versa. That's got to be less confusing than "well, FR is D&D... but D&D is also Eberron, and Dark Sun, and..."
So I'd expect the focus on FR to continue for the foreseeable future, because it's almost certainly easier to sell that way. (Though I would
also expect to see appendices on transplanting the material to continue, except where that material is especially tied to FR (like OotA).)
Oh, and I certainly don't expect to see FR swallow Eberron, Greyhawk, Dark Sun, or anywhere else. Because there's just no need, and no good reason to weather
that backlash.