Does it fit though? I can appreciate wanting a streamlined and well thought out cosmology, like I said, I've made plenty myself (I like to think my Sertorious setting is pretty well organized cosmologically). But I also think D&D isn't even attempting to make something like that. Sometimes you want something that just draws more freely off of real world myth and legend. If you look at a lot of real world cosmologies, they often evolve organically and there are gods and creatures doing double duty with roles leftover from earlier versions. I don't play D&D very much any more, but I used to. And I always felt with D&D you just kind of wanted as many flavors of supernatural as possible so if you saw something cool in a movie or book you wanted to incorporate you could easily find something to tie it to, and ideally you would have multiple options to pick from so it fits what you want in the campaign. Which I think is one of the benefits of redundancy. But it isn't redundancy itself that makes it interesting, it is that by having overlap you are allowed to have greater variety, and not just one lord of darkness or something.
Redundancy isn't inherently interesting, but it can be. The same goes for a streamlined cosmology. The default (for lack of a better descriptor) cosmology in D&D is astonishingly messy because they wanted to incorporate as many real-world mythologies--many of which have mutually exclusive cosmologies--as possible. It's certainly possible to pare that down quite a lot, to pick (or write) one constellation of deities and one narrative to connect them, and still leave the other planar hierarchies more or less intact.
I think it's the variety that causes the overlap, not the other way around. I agree that having multiple ways to incorporate things is potentially a benefit, but I think there's also a lot to be said for having the fictional world be more coherent than the default seems to encourage.