That may not be what you would like to read, but do you honestly think that enforcing policies by forcibly throwing off passengers holding valid tickets because TOCs don't have enough capacity is a good idea?
Who said anything about throwing off people holding valid tickets? I certainly didn't, so can we dispense with that strawman please.
My comment was merely saying that it is a shame we have to rely on something that is horribly unreliable. If we could rely on the goodwill of other people, we wouldn't need to bother with laws, regulations, or authoritive forces. In the case discussed here, then it depends on whether the person in the wheelchair couldn't board at all, meaning there really was no space without turfing other people off the train, or whether there was space for the wheelchair to get into the carriage, but no room to move to the disabled space. in the former case, it is hard luck, the wheelchair-bound person cannot board if there is physically no space, and it is not fair to evict people who have paid for a ticket. If the latter, then there is a logical solution. Ask enough people to get off the train for a short period of time. Using the now cleared space, move the wheelchair to the designated disabled space. Ask the passengers to get back on the train, using the space near the door which was formerly occupied by the wheelchair as standing room. Just a minor bit of shuffling, everyone gets on the train, no-one is thrown off and left behind, everyone's happy. To maximise cooperation you would probably need an authoritive figure to explain the plan and assure people that no-one is getting left behind.
Ultimately, if you are going to have a rule which states vulnerable passengers have priority (be that seating or disabled space) then there needs to be a way of enforcing that rule, otherwise the rule is pointless and has zero authority. That is why a lot of motorists violate traffic laws, because they gain from it and mostly get away with it, because there is no-one around to enforce the rule. It has got to the point where a minority feel entitled to break the rules when they feel like it, because the consequences are externalised onto others.