I don't think the TOCs should be under any kind of obligation to accept a delayed flight as a reason for allowing flexibility in an advance purchase ticket, but I do think it's poor business sense not to.
I say this because nobody, or such an insignificant amount of people that it doesn't matter, ever land at an airport and then think to themselves "nah, I don't want to go home/to my hotel, I'll hang around here for a bit". So if the aggregate reliability for all airlines is around what, 85% I think? then that is the amount of people who will be on the train they are booked on. However, the amount of business the TOCs will lose from people (like me) who are not prepared to risk it with advance tickets and think standard fares are too expensive compared to the car, isn't worth the amount they can excess on the 15% who don't make their booked connection.
All it would need is a simple system that, say, you had to upgrade your ticket on the day to a flexible one, but you could apply for a guranteed refund from head office minus an admin fee of £5. That would mean people weren't put off from travelling by train, but yet of the 15% who had to upgrade, not all would apply for their refunds as many would forget or couldn't be bothered. But they could not then take their frustation out on the railway, as the onus would be on them to get their money back.