Not sure that would be popular, no Advance fares, no return fares, no simple ticketing from TVMs and queues at ticket offices. Oh, and probably no season tickets either.
I think you misunderstand me. By "at the point of travel", I don't mean that you have to go to a ticket office/machine and buy a ticket for that train. Rather, I was thinking of having something on the platform or train which deducted credit from an account you have as you travel.
As to your claims:
It would not be popular at first. Some people would be charged more (and I'm not just talking about those who expect to be able to clog up trains for hours at a time to take a 60 mile journey), and they won't like it. Some people will have difficulty with the technology, and they won't like it. People would get used to it, though.
Advance tickets would be unaffected. You commit to using specific trains at point of sale, so the price is known and the company can sell you tickets.
Return fares would go, as I don't envisage the traveller ever telling anyone that he or she wants to go back that way.
TVMs and ticket offices won't sell walk-on tickets, although they may still provide a way to credit your card account.
Season tickets would probably be replaced by something more like the zone-based tickets various PTEs promote; you buy a ticket which allows any journey between a particular set of stations, and use it like that. Season ticket users are more likely to complain than other groups of rail users simply because they pay so much at a time, so I'm sure the industry would work something out.
I'm sure there have been previous threads on this subject, though.