Given that TVMs can't issue all the tickets someone requires, I don't understand why the permit to travel machines have been removed from so many locations.
Yes, it would allow someone to buy a 5p ticket and perhaps never pay the other end - but if these were recorded in some way (i.e. the tickets that were 'upgraded' later were recorded), it might just tell the TOCs how many people wanted, but couldn't get, a ticket.
Have no permit, and you have the book thrown at you (assuming it was working).
But why can't TVMs issue all tickets? Well, we've had that discussion many times and it seems to be on the assumption that everyone will buy discounted tickets or point-to-point tickets one stop after the origin, and one stop before the destination to save money. But, that's when onboard RPIs should get you - and, again, you have the book thrown at you as it's fraud. No penalty fare, no warning: court.
Society these days only seems to care about things when they really do get punished, but the current system turns the innocent into potential criminals as we seem to have to cater for the fraudsters.
TVM software could surely be updated to require someone buying a Gold Card or YPRC discounted ticket, or ticket extension, enters their photocard/railcard number on an on-screen keyboard that is then printed on the ticket. Along with a on-screen disclaimer that reads out the key T&Cs (so there's no way of arguing you didn't know them), it gives RPIs another way of keeping some control.
Perhaps set gates to 'seek assistance' for people who have bought a ticket extension or discounted ticket from a TVM - then make any checks there and then - and punish those who tried it on severely.
We should have a system in place that allows ordinary passengers to get what they want and travel with relative freedom, and punish those who abuse the system. Not think of all the clever, or not so clever, ways to avoid paying and then remove these methods to the detriment of everyone else.
We also need to re-think the way we issue tickets. If we still want paper tickets, what about a newer type that can store more information (surely the gates themselves would only need updated software?)? In Barcelona, information is physically printed on the rear of the ticket (useful when I killed my 'T-10' 10-journey ticket after putting it near my BlackBerry accidentally; it meant they could issue me with a manual 'T-8' ticket) and in Germany you must validate your own ticket, which proves where you got on (and there's a tough penalty if you don't).
Maybe we should look around to see other ways of doing things, and get the ball rolling to move people over to smartcards. With many new mobiles coming with NFC, you'll even be able to buy a train ticket on your mobile app and transfer it wirelessly to your train ticket.