The way I see it, email is the simplest and cheapest way. Although I wouldn't like average Joe to casually login to their email account on public machines.
But then, chances are the email provider will have 2-way authentication, so it would be a bit more work to get to the account every time on a new, public machine, but I guess that wouldn't be that often and would be preferred to the current way of doing the same thing.
SMS creates cost which some sites couldn't bear and the app route creates greater cost of implementation (to be a true replacement - or should I say truly disruptive ;) - it would have to be cross-platform, IMHO).