I don't see that this "locks you into" the "official" world so much as this is "added value that the official world offers you". What was arguably missing, of course, was good guidance in the DMG about both using and developing variants on this stuff for your own worlds. But then, advice about a lot of the "new paradigm" in 4e was absent or stunted - maybe because it was new to the designers, too?
Eh, there was no more guidance about how to use the cosmology in AD&D than there is in 4e AFAIK, but it was a lot harder to do much with for the reasons Pemerton states. Mostly it was just that it was hard to see how the GW cosmology was relevant in an ordinary sense to the inhabitants of the world. All ordinary people in that cosmology could possibly do was hunker down and hope nothing busted loose, they were both irrelevant to and had no stake in any sort of conflict or etc that might be going on. The universe was basically an unchanging reality that bore little relation to even the PCs and which didn't really help you frame conflicts. Later on they added in the "Blood War" clearly for exactly that reason, but again the PC's world was at most some minor side front in that. You could of course engineer more out of it, but you had to construct the whole conflict and plot.
Compare this with 4e where the relationship between say Moradin and Demogorgon is going to be PERFECTLY clear and they have an ongoing active conflict in which the stakes are primarily the 'natural world', which is the focus of that conflict. A conflict which is also built around the theme of order vs chaos, which is THE central theme in all human myth and religion when you get right down to it. The story telling potential is rich and the situation fully charged and ready to go. AD&D gives you the pieces of a gun to assemble. 4e gives you a cocked and loaded gun that is ready to fire. Of course you can always reflavor the 4e material, Moradin can be replaced with some other thematically appropriate god, and so can Demogorgon if you wish, but the overall setup remains, order vs chaos, the world at stake. A narrative of mighty conflict based in the founding of the world and destined to play out until the final apocalyptic end of time (again you can adapt all this as much as you want).
Obviously if you really want a VERY different sort of cosmology and conflicts than 4e provides then its material isn't maybe any more helpful than the GW cosmology material was, but I have to ask, when is it LESS useful? Given that 4e's rules really don't worry about the cosmology outside some very casual explanations of some magical effects, there's no big problem with just rolling your own thing.
I suppose you could say 4e lacked some world-building advice for creating a different cosmology and world, but there was no such thing in AD&D either. They gave you a cosmology and just assumed you used it. There isn't even a suggestion about what the gods are or how they work in AD&D. 1e in fact slaps you with a single 'priest' class that has no options and a pseudo-christian flavor to work with, and the paladin, again with the pseudo-christian flavor. Later on the various settings added gods, and they got more embedded in 2e's lore, so you had something to start with, but even then it was mostly bad guys, the good guys were singularly lacking in definition and motive. 2e had at least the 'priest' class, but then they were strictly weaker than clerics, sort of undermined the whole concept. I'll take 4e's world-building advice any day! It was mostly example and template, but they did hand out some perfectly good ideas in their example story and campaign arcs and some of the DMG's later chapters.