My perspective is that now more than ever, 6E needs to be significantly different from 5E. If I can play and run 6E with my old books, or with a third party game, why would I buy a new PHB or DMG? If I can run existing monsters unchanged, why do I need a new MM? And if I can do all of this with third party games that might also be more to my tastes, why wouldn't I just do that? If it's compatible to the point where the books are interchangeable, why would I want to buy it?
It's the new shiny, and there will be a lot of people who do buy it just for that reason.
WotC has released a fully compatible edition upgrade once before with Essentials, where you could use your original PHB with the Essentials books entirely, and that edition didn't exactly set the world on fire.
I don't really know what the future will bring, but WotC (and TSR before it) always talked about compatibility for edition changes and with a really modest exception (4E Essentials) they never really were when the books were released.