Now don't get me wrong, I don't think 6e will be a radical reinvented edition, but I do think there will eventually be enough revision (or calls for revision) that it will no longer be 100% backwards compatible. They may want to tackle issues with rests or multi-classing or such down the line and that will break the clear 2014 - line of compatibility, even if we're not talking as radical a change as the last editions were.
Oh, I'm sure that 10 years from now, when D&D (whatever revision) comes out, they likely won't be looking too hard at 2014 for compatibility. Depends really on whether any of the 2014-2023 material is still selling. Even now, Hoard of the Dragon Queen is selling very well. Unbelievably well for a module. Jaw droppingly astoundingly well for a module that's close to ten years old. As I type this, Tyranny of Dragons is sitting at 1100 (ish) on Amazon.com.
That's unfreakingbelievable. That a module, any module, would still be selling in significant numbers after all this time? :boggle:
So, if ten years from now, Tyranny is still selling at the same level it is now, I guarantee that the next revision will be very backwards compatible. As far as I'm concerned, that's how you can measure how far they will deviate from 2014. When 2014 supplements stop selling in significant numbers (say drop below 10 000 on Amazon for example), then they'll make bigger changes. But until then? It's going to be middle of the road baby. Smooth as silk sailing for as long as they can ride that train.
I mean, heck, I presume that 2014 PHB's will drop in sales after 2024. I would assume. But, then again, maybe they don't. Maybe they keep selling both versions. The 2024 would presumably sell better, but, who knows how long a tail 2014 will have. And every sale of a 2014 PHB is gravy by now. So, in 2034, when the next revision hits, watch all the pre-2024 stuff, see how it's selling. That will tell you how much change to expect.
DId a little noodling around and noticed something. The worst selling WotC book right now is Storm King's Thunder. At 6700 (ish) in all books. Let's put that in perspective. The best selling Pathfinder book on Amazon is Rage of Elements. It's sitting at 26000 (ish).
I mean, for all I bitch about wanting new stuff in D&D, I do get it. Why on earth would WotC rock the boat? Spend all that money on research and play testing and whatnot, take all that risk, for what? It's not like a heavily revised 5e would sell better than it already is. Maybe it would, but, let's be honest, probably not. So, yeah, so long as the market keeps growing, or hell, even if it plateaus but doesn't drop? Yeah they aren't going to do ANYTHING to rock the boat if they can avoid it.