Your Planescape-slanted explanation kinda rubs me the wrong way, especially when it comes to Eberron's cosmology. We don't need some sort of grand unifying explanation as to why the cosmologies are different anymore than trying to explain Star Wars' concept of the Force into Star Trek. It strips a lot of the individual charm out of settings, again imposing Planescape norms on other settings.
I'm still failing to see where anything is being
imposed. I tried to be pretty clear that it's
my theory that I use in my campaigns. When someone plays Eberron, if they don't want Sigil or any of the Great Wheel to exist, they just snap their fingers Thanos-style and they are gone! Nothing imposed.
Take your adding the Force to Star Trek example. If someone published a book specifically about how to combine Star Wars and Star Trek, then I would expect an explanation of how Vulcan mind melds are really Jedi powers or some such. For some fans that could be really interesting. But for Star Trek fans that don't want them mixed, nothing has been imposed on them. Vulcans aren't suddenly ruined. They can just ignore that book and go on happily enjoying their Force-free Star Trek as long as they like.
If my explanation rubs you the wrong way, don't use it. *shrug* I'm not imposing anything at all. When I play Eberron, I very well might not use it either.
Prior to 3e, TSR never really considered alternatives. In 3e and 5e*, WotC has been very clear that you can do whatever you want, and some settings will be very different. 3e Manual of the Planes, FR, and Eberron were very clear in discussing entirely different cosmologies. But prior to those, the concept simply wasn't really there yet. Prior to 3e, they also didn't have sorcerers. To say that the 2e PHB imposed wizards rather than sorcerers onto players would be a really weird use of "imposing." Same thing with the Great Wheel prior to 3e. The lack of alternatives has nothing to do with Planescape and is simply because they focused their creative efforts elsewhere.
The existence of Planescape imposes nothing on Eberron or any other setting. Just ignore it. All current 5e books have tried to be open about alternatives. In the DMG, they even specifically call that chapter "Creating a Multiverse" for good reason. Unless Spock talks to Yoda in the next movie, some people geeking out about mashing up Star Wars and Star Trek or even the existence of any books detailing that, imposes nothing on either of those two properties. Even if a book was published
bizarrely mashing up Star Trek and a rather mismatched property, it's easy to enjoy or ignore it on its own without it imposing anything on the rest of Star Trek.
So I guess I'm just confused on how Planescape having it's own explanations and ideas imposes anything on any other setting.
* I'm not familiar with 4e very much, so I can't speak to that. I certainly got the feeling that 4e did have much of the problem you worry about with imposing it's World Axis cosmology and history onto all D&D and trying to erase the alternative, but that could just be from my unfamiliarity rather than what 4e actually was.