I agree with your last sentence, to a point. I think eventually, viewing on-demand will be the norm. However, there are several obstacles blocking the progression.
1. Cable companies are gonna fight tooth and nail against this until they can demonstrate that they can make more money with a new business model.
2. Content providers are going to balk at this model unless they can make more money with the new model.
3. Consumers won't adopt this approach until it's dead easy to do, and the pricing structure isn't too outrageous.
Here's my dream TV system. A single (non-computer) device that is instant-on and zero-downtime that can record every channel that I subscribe to (via multiple tuners). It can also access on-demand programming from the cable company (or sat provider) as well as the internet. It would also be able to access my photos, music, home videos, etc from all of my computers. This device could be located anywhere, but it would be connected to my network.
I would have a small device located at each TV that can "grab" a tuner from the central device and watch live TV. This box will also be able to stream all media that the central device has access too.
This is a true "whole house" system.
This will never happen as there are too many competeing interests fighting from multiple directions. The best you can hope for is a system that meets as many of these wants as possible.