Migrations are something that any user could want to do, and should be able to do . Moving something like Voice Mails and text messages, while not commonly needed - i've personally never cared about migrating them - shouldn't be considered outside the norm either.
Does Windows do something that nearly no other OS does in the requirement/user of extensions to know what programs to use to open a file - yes, is this now a legacy problem, sure, doesn't change the fact that people want to migrate every day from iphones to androids and vice versa. Just this year 4 of my doctors switched from iphones to galaxy notes, mostly at the behest of their teenage children. Frankly I couldn't believe they were doing it!
So Personally, yes I think migrations are a highly common thing. That said, if apple doesn't want to support that ability, that's their prerogative.
I'll agree that backups don't equal migration tools.