Paizo already has a loyal fanbase, and they've pretty much said that they'll never again put themselves in a position where their own products depend on another company. I don't see it ever, ever happening. At most, I can see them possibly doing a small line of 5E material or conversions alongside Pathfinder, but going full-on (or even mostly, or even a significant minority) 5E? I think not much earlier than the heat death of the universe.
There's a 5e SRD now, so Paizo can release a PF 5e Core Book whenever they like. All it would really do is make PF compatible with 5e. It would be taking advantage of 5e's success, while keeping your own advantages (more output, more crunch).
Could they? Yes. Will they? Not for the forseeable future. If PF RPG sales start tanking and people cross in droves to 5E, then they'll probably have to. The core Paizo/PF fan base is pretty large, satisfied and well services by Paizo & 3P Publishers. I think we'll see a PFRPG 2nd Edition first.
Here's the thing though - what does "cross in droves to 5e" even mean? They're gonna buy 5 copies each of the core 3 and 4 AP hardcovers? You buy the core 3 and you've got practically everything but the adventures for 5e, and that's if you're a DM. If all you do is play, you buy the PHB and you're done. In other words, people who want to spend money on 5e player-oriented content have nothing to spend it on. They've had one book in a year and a half. Sorry, 1.5 or 2 books, depending on how you count the Sword Coast book. And the next book WotC is putting out? That's not for players, either. And neither will the one after that.
Instead of simply being content with the customers they have, Paizo should be trying to sell product to those people who want to spend money on 5e, but have nothing to spend it on.
Ultimately, it all depends on money. Is their current system making them enough money to remain profitable? If not, they may explore other options like the New OGL.
More money's always better. They can choose to unify the player bases into one base, and sell to a bigger market, or they can choose to stick to the one they've got out of intertia.
Basing PF2 on 5e is two moves for the price of one; you not only re-boot your whole line (which you kind of need to do because surely it's at or near the saturation point), you also tap into 5e's player-base. Sure, that may anger some players, but how many are going to turn their backs on a unified PF/5e system/player base and go looking for something else? As long as they get their crunch and their APs, are the Paizo customers really going to flee in droves?
They can continue to make their AP PDFs in both flavors, of course. Something tells me those guys know their business, and streamlining the process of creating adventures and encounters for 2 systems at the same time is something they can handle. Printing is more complicated, of course. They can give both systems a go and see which one sells best. They should at least be trying this.
The only good reason I can think of not to do this is if Paizo brass has no intention of doing a PF2, not even a 3.8e (or whatever) version, deciding instead to keep everything "evergreen."
Let's take a good look at how Paizo got to where they are today: SRD, OGL, make good products. Is "never again" really how Paizo views what happened when WotC dropped the ball with 4e? I'd have thought, "again, please" would be a more constructive response.
What have they really got to be mad about? They ate WotC's lunch, and now they have a chance to do it again. With the kicker being, WotC doesn't really even seem to want to eat it, anyway. Win-win.
I think we'll see a PFRPG 2nd Edition first.
Why not kill 2 birds and build the 2nd Edition of Pathfinder around the 5e SRD?
