I can see the point, but I think the benefit isn't as big. You can create very different cosmologies and still introduce a link between them - as long as you have a cosmology at all, anyone wanting to link them can still say "oh, and then there is this metaplane where you can, if you're lucky, visit other parts of the multiverse with different cosmologies...".
Not every setting needs a unique cosmology, but some can benefit from it - especially those that might already have interesting, pre-existing cosmologies.
Many can benefit from their own personal flavors.
I think something that would be telling is IF the idea of Spelljammer was built in with the crystal spheres that would allow greater deviation from PoL without needing to interact with the Astral Sea.
I have been thinking Plane of Knowledge from EverQuest every time I think Astral Sea, and can only see PoL providing for the single universe theory within the system.
Might get lucky and get some diversity, but that would be up to the designers. I think Eberron might be the first to shine some light on it all.
Was Takhisis mentioned about being in the Draconomicon? Being a god that would surely be something that says more cosmology is possible with her dragon form existing in there.
But it seems PoL and the PHB cosmology is built around a combination of Eberron and the Realms, which makes for an odd outlook on future cosmologies to me. We can only hope and wait.