I *just* pre-ordered Princes of the Apocalypse. I don't really want the adventure and am unlikely to run, but the idea of new monsters and a hard copy of the PC options is too appealing to pass up.
So the strategy might be working. By combining the two books into one, the value of PotA becomes higher. We know that the two Tiamat adventures combines is 192 pages, and that PotA is 256-320 pages. So there's 64 to 128 of non-adventure material in that book.
We also don't know for sure why the Adventurer's Handbook vanished. It's very possible the content was lacking or Sasquatch Games (being somewhat new) just wasn't able to fully deliver on time. Or WotC looked at the $50 price of Princes of the Apocalypse and decided it needed more *oomf* to sell at that MSRP.
I agree with the strategy that fewer more important books is a good plan. But I'd still like more books than we're seeing now. It sucks right now, but I'll be thankful in three years. After only three campaigns over four years, Pathfinder has become super unwieldy to me. It's a matter of balancing the short term desire for more content right now please and thank you versus the desire not to have the game spent in four years.
And as the Unearthed Arcana articles get going (and hopefully 3PP) the lack of content will be more bearable.
Two accessories a year would be nice. But if I get two 320-page adventccessories per year, that's fine too. There's a LOT of reasons why that might be a good idea.
It's likely cheaper for WotC (and me) than a 192-page and a 160-page book. The economics of publishing mean it's better to sell more copies of fewer books for fewer total sales then it is to sell fewer copies of more books for more total sales.
Because the book appeals to multiple audiences, it might be purchased by both DMs and players, increasing the audience and potential sales. A little like what they did for the Dark Sun and Neverwinter books.
Having a book be split between crunch and adventure means they have more wiggle room to make the adventures larger or smaller depending on the needs of the story. They apparently cut a LOT out of Rise of Tiamat. And if a lot of crunch is needed for a story, they can make the adventure more svelte.
It's easier for people running the adventure, since they have everything, and don't need to get a second book for the spells and races and powers needed.
And since I bought the product, the sunk cost fallacy comes up in a couple ways. If I bought the book for the crunch, I'm encouraged to run that content. And if I bought the book for the adventure, I'm encouraged to allow that crunch. And, if I'm a player that bought the book, I have this adventure I paid for encouraging me to try DMing.
The format also groups and themes the expansions content. You have the elemental content bundled with a related adventure, which you can use or ignore if running a different campaign. This makes it easier for the DM to say "PHB and whatever is related to the adventure" limiting bloat. And it potentially makes it easier to find content. You don't look for psionics in Princes of the Apocalypse and you don't look for elemental power in something like Brain Stealers from Below.
But we'll see. WotC seems to change strategies on the drop of a hat.