When I waqs far, far younger and far, far newer to DMing I used to care a great deal about using the TSR backstory because it felt I was playing the "real" D&D. Looking back on those times I understand that I simply lacked the confidence of my own creative talents as a DM and wanted my campaign validated by keeping it by the book. My players ever expected this of me.
Then as the years progressed I realized that every campaign setting was someone's homebrew. Dragonlance was Weis and Hickman's. FR was Greenwood's homebrew. Greyhawk was Gygax's homebrew. And so it goes. All the best settings were someone's homebrews existing merely in another's imagination until set to print by TSR and WoTC.
It goes without saying that a DM has to change the backstory or even dramatically alter a great many things in oder to create a setting that isn't just like every other setting. So what is the big deal if WoTC changes the backstory to succubi or dump the Great Wheel unless you are simply cracking open books and DMing the utterly colorless "core" setting as is. This is supposed to be Greyhawk in 3.5, but Greyhawk fans know the violence done to their favorite setting by making it the vanilla backdrop for the game and home to every idea from every splatbook by default.
I don't know any DMs who have been DMing for a while who do not use a published setting such as FR or Eberron or create their own setting. I don't know why is matters what WoTC does in regards to succubi, the nature of demons and devils or whether or not the Great Wheel still exists if each setting diverges strongly from so many core assumptions anyway.
In my experience no homebrew or published setting in the 3.5 era has been slavishly following core assumptions so I would argue it is irrelevent what WoTC does or does not do to the backstory of D&Ds critters, planes and mythos.
Rules changes....well they can be a problem, but fluff....no problem at all.
Sundragon