Given that I spend my time kicking people out of First Class who INSIST that Standard Class is full, and I then walk through the train and find anything up to 130 empty seats (last Friday), and quite often these same people are standing in First Class and not making any effort to find a seat anywhere on the train (but still insist that they won't move unless I find them a seat) then I think that this is the greatest load of bollix I can think of, and just pandering to the "victim" culture that is encouraged.
If people want to travel in First Class, pay for it. If people want a turn-up-and-go railway, then one of the down-sides of it is that they may have to look a little bit harder to find a seat. If people want a guaranteed seat, then make a reservation, or get a quieter train, or even walk through the damn train and look for one.
If my trains are crush-loaded then fine, I will declassify First Class. If somebody is at need of a seat (elderly, infirm, ill, pregnant, kids, disabled) I will often sit them in First Class if no other option. The problem with this sort of thing is that these are not going to be the people who will sit in First - it'll be the pushy self-important commuter who runs on-board at the last second and then decides that it's too far to walk down the train to find the empty seats, and think "I'll have a seat here, I'm entitled to it".
To summarise, "If you are on a Standard Class ticket and would like to sit in First Class, you should have worked harder at school and got a better job" (my announcement leaving London on my last day at work).