Still doesn't really save you unless you can export all of your data into some open data format that can be read hassle free by other services (oh and you need a guarantee that the company won't just kill their export functionality when they get bought by another company etc).
You also have to ask who controls the browser, this is going to ultimately be whoever controls the OS. iOS won't allow alternative browsers, not sure if Windows RT does?
So there's nothing really stopping the OS vendor with enough market share deciding they won't allow certain HTML5 / JS functionality incase it interferes with native app sales.
It might seem the case that one or two big ecosystems eat up everything else and choosing which ecosystem you live in becomes a major life choice. At this point computing and media have become a natural monopoly.
Of course then follows the "open" renaissance.