Put me down for saying no middle ground is possible. If you have new print product coming out at any rate higher than zero, you WILL have the wall of books. ... So, I will say this for the current release paradigm: those slim adventure books will hopefully keep the wall of books a little bit lighter when we eventually get there.
Actually, it's not inevitable, especially with the current adventures-only approach: all they need to do is let the older adventures drop out of print at about the same rate as they publish new ones. If "Tyranny..." ceases to be readily available at the same time as "OotA" appears, the shelf of books in the FLGS will change from having the current 7 hardbacks to having 6.
Bear in mind that the "wall of books" is
only an issue for potential new players walking into the FLGS for the first time and who therefore don't know where to start. People shopping online who don't know have Google, which makes every question an easy question (or perhaps the WotC site, though that tends to make even easy questions an exercise in frustration). And it doesn't matter how many books you or I have on our shelves, because in that case it's because
we've chosen to put them there.
It is considerably harder to avoid a "wall of books" if you publish non-adventure supplements, of course. 4e did reasonably well with its setting support (only), in that each setting got a very limited amount of support and then stopped, and also only supported a given setting for a limited time. Though they didn't let the older settings become unavailable.
(And the current model of the online supplement for the current storyline is reasonably good for this, too, especially since the
next storyline will most likely have its own supplement and, importantly,
won't allow the use of the current supplement in AL games.)