Someone posted on another thread awhile back that the wheat buns tend to dry out quickly once the package is opened and that's what causes them to crumble. With thousands of people eating at WDW I can't imaging theirs to sit around going dry. When I get a Subway sandwich, it's always on their 9 grain wheat bread (so I do like wheat bread) and have never had a problem. I'm thinking it has more to do with an inferior product that plays into budget cuts at WDW than the actual type of bread used. My first encounter with a "crumble" bun was a few years ago at the Backlot Express in the studios. I have always eaten my hot dogs plain, so there's no issue with condiments. The bun literally fell apart in my hands. In any case, there are no options avaiable. Also, since I don't use condiments on a hot dog, there is nothing to mask the quality (or lack of) of the hot dog and that's when you really notice how bad they have gotten. You may have noticed that the CS hot dogs now come with chili unless you ask for them plain.
The quality of the burgers and hot dogs has been creeping downward for quite some time. I can remember when even a CS burger or hot dog was pretty decent, probably not as good as your favorite chain products, but good enough that you didn't mind spending a few extra dollars. The changes were subtle for years, but in the past few years they have been anything but subtle.