You could also argue that Safari on desktop is more likely to be updated as well since it is generally accepted that Mac OS X has a higher adoption rate of new versions since:
1. enterprise use of Mac OS X is pretty small, and its generally enterprises that are slow to upgrade OS/browser, etc (albeit for generally valid reasons)
2. OS X upgrades have been free for some time now.
XP reigned for over a decade. No iOS version will be widely used for nearly that long.