Pons, I don't think flexibility is gone. Picture books like Sophie's Squash and Yard Sale and others are still getting published, with word counts of 600 and 700 or so... The idea that they have to be 200 or 300 words, which is widely mentioned "common knowledge," just isn't entirely accurate, or is based on books by author-illustrators, which are different.
And as someone else commented, word count isn't the only thing that matters in picture books. Overall, I see more variety (in approach, subject matter, style, and so on) in the picture book market today than existed in the "golden age" of the 50's and 60's (or whenever we think that was...). Let's celebrate that.