As a parent I used to shy away from very text-heavy picture books for my daughter (ie those that are well over the 1000-word mark) but that's because I always read her three books before bed, so a very long story would get her to bed late! Now, though, she seems more able to handle longer texts - she doesn't need a new image for each new paragraph. And, funnily enough, she has started rejecting picture books with just a line on each page, saying they're for babies. This, despite the fact that some of her favourite PBs feature just one line per spread! (Chicken Cheeks is one example. Mo Willems pigeon series being another.) I definitely like 'getting my money's worth' from a PB and being able to spend time reading it is part of that – which means more than just a couple of lines on each spread UNLESS the book is hilarious, fantastically clever or something equally spectacular (as the above examples are). Getting my money's worth might also be down to the illustrations - anything by Caralyn and Mark Buehner, for example, is value for money because you can spend hours hunting for animals/forms in the clouds/shadows etc.
I think it's far harder to make a picture book with so few words but I think Mr Joe and Mrs Jo Public just can't see that so they're probably less likely to splash their cash on something that, to them, looks as though it's not value for money.
Blimey, what a waffler I am. Not feeling very succinct today! But in brief, I agree with your belief!