I get why you might want Eberron and Athas to be remote, but that shouldn't mean they are never accessible.
I feel like part of Eberron's appeal is how integrated everything is. In other settings, guidance frequently was along the lines of "cross-planar adventures should be higher tier than starting adventures" (obviously, except things like
Planescape) but when they showed up it felt like an afterthought. On Eberron, manifest zones ensure players will be aware of planar shenanigans early on, and keep being aware of them. Adding other planes into the mix kind of messes that up.
For instance, in
Eberron, researchers have tracked some of the souls of departed, and they went to Dolurrh; it's not exactly clear to them why or how and it's difficult to research. The foremost researchers on the phenomenology of death were in House Vol, and there aren't any left anymore. In
Planescape, souls go to an afterlife for the most part based on the god they worshipped. There's a lot of in-universe lore about how and why it works that way. It fits in pretty well with Greyhawk and the Forgotten Realms, but it drastically reduces a part of Eberron's unknowability. It changes its cosmology if both
Eberron and
Planescape coexist.
Really, deep down, I don't want Wizards of the Coast to cross the streams because of a particularly obnoxious FR fan who sat at my Eberron table at a convention and really, really,
really wanted to bring Elminster into Sharn.