In any case, and in summation, WotC did not change the planes as change for their own sake. Everyone knows their reason -- make the planes more playable at all levels -- even if they don't agree with it.
Some folks feel proprietary about what's come before, as people do about stuff that they care a lot about, but WotC has said at least as early as their preview books that they intend to take a greatest hits approach to D&D lore and incorporate it into 4E (which I guess is the reason for the tacked-on ending to Expedition to Undermountain, come to think of it). They are creating new stuff, even if none of it has particularly set the world on fire yet. (I don't remember anyone being particularly excited about Keep on the Borderlands when it first appeared, either; people mostly loved it in retrospect.)
There are obviously plenty of people who liked the old cosmology and didn't find it problematic at all. There are also plenty of people glad to have something new. Neither group is wrong: These are opinions we're talking about. (And most people on all sides are very nice about it, if passionate.)
Personally, I'm glad this is all happening in the Internet era, rather than before, because that means nothing really dies. If Mystara is going strong, and there are competing Al-Qadim fan hubs and Dark Sun keeps soldiering on, then Planescape is unlikely to vanish either, especially with official PDFs out there.
And, for the record, back in the 2E era, I felt my skin crawl with all the "Return to" modules "ruining" the 1E and BD&D classics. Now it turns out that people adore several of them. It's entirely possible that the same process in 4E will likewise turn up a few new gems. I think planejamming seems like a potential gem, for instance.