I'm always a little surprised Paizo hasn't done something for 5e. Kobold Press, Green Ronin, and even Monte Cook Games have all embraced 5e. (And Mr. Cook was really burned by WotC after he returned.)
D&D isn't just the biggest RPG. It's the biggest three RPGs. Pathfinder was #4 behind base D&D, AL D&D, and Critical Role D&D. Even 3rd Party books by Kobold Press were outselling the Pathfinder Core Rulebook on Amazon. A Paizo published 5e book might easily become their best selling book by a wide margin.
Yeah, it makes sense that Paizo wouldn't want to delay or slow down their releases for Pathfinder 1 or Pathfinder 2 to make a 5e book, but since they'd have to rely on freelancers anyway for the design—and much of the art, planning, and such might already be done—it shouldn't impact their release schedule.
(Really, the last couple years of Pathfinder could really have used a slower release schedule.)
Even if they didn't update a full AP, they could do a monster book. There's an eff-tonne of Paizo-specific monsters and Golarion lore. And all the art and page assets would already be done.
Or they could sell conversion PDFs, which allowed you to play their existing APs, selling them like they did the digital map packs.
I understand that Paizo doesn't want to have their business tied to D&D and another company, like they did during the days of 3e. But there's a big difference between tying a single product line to D&D and leaving the rest of their business independent. Especially when said product line could radically increase their profits, funnelling more money to the APs and Pathfinder fans. If even a fraction of the massive D&D audience bought their APs because of a professional conversion, it could double their sales.
Honestly... it *almost* feels like a pride thing. They're still pissed about what happened during 4e and feel a little stung from losing the #1 spot so they want nothing to do with 5e.