I'm not talking about right now, I'm talking about what happens several years from now when suddenly there are games that are all but unplayable (or completely unplayable) on first generation Steam Machines.
I'm sure the plan for that is the exact same as the plan for the current situation. Steam machines have specs, the steam client can read those specs and tell you if a game is playable or not. It's not as though gaming on PCs is a new concept.
Why wouldn't the plan be "buy a new one" like it is with consoles, pc's, and every other electric device when they either break or the newer models are that much better?