From the start of Maemo, Nokia apparently intended annual major-version upgrades to the OS. I find that to be a reasonably rapid OS release schedule. Heck, I still see Windows users bashing Apple for churning out Mac OS X releases that frequently.
Swapping out GTK for Qt on a device while bringing a massively redesigned UI to the system and all apps may not be as hard to accomplish smoothly as the old libc5 to glibc change, if you remember that, but I'd guess that were it attempted, Nokia would wind up expending multiple times more effort on managing the transition than they did on actual design and code.