MS commits to Surface with Windows RT for at least 4 years.
The key here is 'with' RT, implying that the surface tablet, or surface range will continue with or without the OS.
Still, 4 years must be enough time in MS eyes for the platform to succeed or fail. Windows Phone is proof they dont mind the long haul and a statement like this serves to assure early adopters and indeed devs that the platform will endure despite early criticism and a lack of support.
The key to its success however is not only oems bringing the hardware, but just how quickly companies get their software moved into the windows store.