Pulling on this thread a bit, when I get a new set top box, most of the apps have a url or a QR code you can use to copy your session to a new machine. You could just do that for all machines and have no cookies at all. iOS even has some ways to make that a bit easier.

For some apps this would suffice. But for apps we use to live our lives, what do you do when someone has no access to their devices and they need to log in to a fresh device? Some apps will need passwords, or some other mechanism to prove you are who you say you are. But then you can leverage those apps to get back into your other apps in the case of, say, a fire.