I seriously doubt that's where it's headed at least not in the way you describe. The trend is towards on demand streaming. Even HBO who I thought would be the last is one of the first with their no landline subscription needed access to their on demand service.
There may be usage based billing, but a set fee makes more sense so that even if people get really busy and barely watch anything for a while, companies still make their money.
Some cable companies are looking into turning essentially every channel into nearly on-demand with features such as rewinding any show back to the beginning even if you didn't record it on your DVR or have it playing with the real-time buffer to go back on.
The future is trending towards what customers want and when they want it. The holdouts run the risk of having an upstart that full-on embraces new tech replace them.