The reasonable scuttlebutt on Harry Potter is also that they are now, since #3, basically just copyedited and put on the shelf. A restraining hand might force Rowling to meander less, but for a big release book it's easier to budget more pages than it is to reschedule the release date and budget more time.

So, if she's late on her submission dates (and I think she consistently is), basically the editors have no choice but to take what she's given them without comment or requests for revision.

I do think there is an upper limit for Doorstop Fatigue, but I don't know where it is, and I don't know if it varies between adults and YA readers.