I assume so, if you, again, think they will have to cover all that in a single product. Again, what price point should WotC choose for that 1000+ page product?
If you are indeed not imagining it, what's in the original box set? Is it not almost completely focused on Sigil and the Outlands? Is its coverage on the remaining planes on par with what is in the DMG?
You do realize that people (like myself) were able to play full Planescape campaigns with just that set - we didn't wait until further supplemental material was released (and at that time, we had no way of knowing that further material would be released). If that was possible then, then that's just as possible now.