This is true only for Business and Ultimate versions. I suppose, that majority of preinstalled systems, which are bought with brand new computers are Home Premium and Basic versions, and licence for this one does not allow such downgrade.
I perfectly agree that this is most unlikely to happen, as OS developers are never forced to optimise new OS for old hardware (because new is always available), even more - they are supposed to force users for buying new hardware. I think, that reverse situation could lead to market stagnation (why would one buy a new hardware, if old one was suitable for new sofware?), and I'm sure MS will not allow such situation to happen.