@Colin as of now, you have no way of telling if Valve won't lock you into their own account system at launch, or even in the future, nor that the device itself will even live up to the hype (which I think it won't, like any overhyped piece of hardware).
Likewise, Valve already has a kill switch on their console-like hardware, they rolled their own OS, they're already controlling that. All it takes is a single update to lock everything down forever.
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