While I do remember the statement before 2e release that all the APs would be playable to level 20, I do understand the reasonings behind 3 book Adventure Paths. I genuinely would not be bothered if it wasn't the fact that most of the 3 book Adventure Paths start at level 1.This causes the exact same problem that many people don't like about 5e in that the modules don't go that high. Unlike 5e Paizo took pains to make a playable system up to and at level 20 for a while.
Of the currently 3 book adventure paths released for PF2e 5 of them start at level 1 and only two start at level 11 (Where the others end at usually). I am hearing from my own players how they are getting tired of playing the 'low levels' over and over, plus wanting to keep playing characters to see them grow into high levels. I am not a homebrew DM, I enjoy making stories come alive not making the story itself.
Six book APs becoming more rare wouldn't be an issue if the three book ones were more evenly distributed high and low. And they don't always need to begin at level 1 or 11. One of my biggest pet peeves about Abomination Vaults which is an AP that I feel is the best one yet for PF2e in terms of story and options is that it starts at level 1 all while heavily suggesting you run your players through Troubles in Otari which ends at level 4 (if you don't give them the final level up). There is absolutely no reason that Abomination Vaults could not have started at level 4 even for a group that is bringing fresh characters to the AP.
And as a side note, I'd be more inclined to purchase more of the single book adventures to slide into my campaigns if their level ranges also were more spread out. Currently (not counting 1e remakes into 2e or ones intended for you play 1e modules like Curse of the Crimson Throne first) the book adventures stand at 2 1st level, 1 3rd level, 1 4th level, 1 5th level, 1 at 11th level, and 1 at 16th level starting level. This leaves a lot of them that can't be run sequentially due to competing level ranges. Spreading out future releases starting level ranges would be greatly appreciated with less 6 book APs on the horizon.