Think of it this way, steam keeps all drivers installed on the machine, starting with a baseline one that it ships with and any future ones that games need. When a game runs they request the driver they need (ie, been tested with) and that's what steam loads and then executes the game. That's how it would work on the current steam box. On steam box 2 you would do the same thing, except if they want to provide a console like experience then what they have to do is test the games on the new machine with its new baseline driver and see which ones work fine. For the ones that do, they are allowed to run on steam box 2 without update. Basically when a steam box 1 game is run on steam box 2, steam would see it's requesting an old driver on old hardware which it no longer has, so it checks it's compatibility list, sees that it's been tested and runs fine on the new baseline driver for steam box 2, and then automatically runs it with that. For games that don't run properly they would have to let the publishers know before steam box 2 got release so that the game could be patched to run properly on steam box 2 before it's release.
That's the only way they can give customers console like stability, and backward compatibility in the process. If they still go with a one driver for all system, then it will be no different than a pc and they have no way of guaranteeing that all a users games will keep working. They do this already with the VC runtime, games use the one they are tested with and a pc usually has a whole slew of these installed by games. They need to extend that idea to drivers if they want this machine to run like a console. Otherwise there is no way to predict if new drivers will cause problems on older games. They aren't going after users like me who will stick to a pc for maximum graphics fidelity, they are going after console users and I can't imagine the typical console gamer would accept that, where a driver update breaks their old games, that would be suicide. I don't think they can half ass this, if they are just going after a rebranded 'steam' pc then personally I think they are wasting their time.