There does seem to be an assumption that many people make that just because things are as they are now, that is evidence that it cannot be changed. Having seen all the big changes that have happened to TV over the last 20 years, this appears to me to be an incredible frame of mind to hold.
On demand viewing seemed to come out of nowhere when cable went digital. Maybe I was asleep at the time, but that took me by surprise - I just discovered. it on the menu when we switched over from analogue.
Just a few short years ago, who would have thought we would ever get a service like Netflix on our TVs? What is more, to have the content available on our boxes, integrated in such a way that we can bookmark its content to appear on 'My Shows'?
You say that none of our public service broadcasters currently operate a mixed model, but that is incorrect. ITV Hub + gives just that choice.
http://www.itv.com/help/itv-hub
As you know, the BBC is looking at creating a website with content from their own channels as well as ITV and Channel 4. If Ofcom allow this to get off the ground, and they have already admitted they got it wrong when they prevented Project Kangaroo from seeing the light of day, then there is no reason why this should not be successful. I would imagine that this project will enable free viewing with ads or uninterrupted viewing with a subscription. There is no reason why such a venture would not succeed.
The BBC's entire output is certainly not based on the assumption they are broadcasting to everyone as you say. It is based on the principle that everyone who meets the all encompassing criteria is charged. I agree that
changing over to a voluntary subscription will lose them a relatively small number of viewers, but a premium subscription offer could take care of that.
You present the choices faced by the BBC either to go behind a paywall or a free to air model. What I am saying is that it could be both, and that would maximise viewership.