You could also look at it the other way: there's also a WILDLY huge market for a tablet+phone in one device -- with the advent of the iPod Touch, the iPhone sales did not suddenly dry up ... nor even level off.

Both options are sound and valid. Hopefully Nokia sees that as well: the right path is not to do JUST one exclusively, but to do BOTH a phone based Maemo platform and a non-phone based Maemo platform.