Funny you should post this because I was just discussing longer PBs yesterday.
I'm under the impression that the ever-shorter trend might be inching back up a bit. I feel as though there's been a lot of very slight PBs lately and readers may be longing for more story. Of course, I feel that there's room for all lengths/kinds of picture books (if they would just kick Dora/Diego/TV/movie books and toys out of the bookstores) but I especially miss some of the longer format.
The general belief is that parents are to blame for pushing their kids toward chapter books and novels at earlier ages. We've heard this a lot the last two years, ever since that infamous NY Times article. But I know my daughter in particular was one who clung to picture books as long as possible...way into 3rd grade (although her teachers would not let her check out PBs from the school library)...and in fact the only longer books she has read have been of the graphic type--Ellie McDoodle, Wimpy Kid, Dork Diaries. This week she finally chose a Carl Hiassen book and I was so surprised, I had gotten so used to seeing her with graphic novels!
Anyway, since little to no market research is done in this area, it's up to the public to change the publishers' perspective on this. I know I've had manuscripts rejected not because they're too long, but because they're for the "older" PB range and some lists are focusing on the younger set instead.
If parents insist on more story, then books may get longer. But it may be tough considering all the apps that now play games and tighten attention spans...
Anyway, it's a debate for the ages. Longer, shorter...why can't we all just get along? LOL