The pc how we know it is goingto die in a few years. A home entertainment system must be everywhere, on every device like tablet (yes Windows rt too) smartphones, smart tv, android set top boxes. This is the future. Not a number of power eating full pcs around the house just to watch tv, some movies and a couple of pictures.
I think Windows RT will be dead long before MediaPortal or x86 PCs. It was conceived when ARM chips had a big energy efficiency lead over x86, but in the time it took to come to market, we now have x86 chips matching ARM in energy efficiency, and being used in smartphones, and so the reason for Windows RT's very existence is no longer valid.