Given the prices that are charged/profit margin achieved by train companies - and the empirical evidence of every single bank holiday ever - I also don't think its unreasonable to expect the train company to put on extra carriages and to have to manage it properly.
They don't have extra carriages sat around. Honestly, there are plenty of incompetent people working at all levels of the railway but the idea of using as many trains as possible is not exactly ground breaking. If they were available to be used, they would be. What you actually mean is that the government should do something about the ridiculous franchising system and invest in new trains and, where possible additional services. Even if there were extra carriages knocking about (or extra trains, most trains these days come in fixed formations, not removable carriages with engines) platform lengths are limited so you cannot just keep adding length to a service. For more trains to run, there needs to be the kind of investment that no one is keen to pay for, and that passengers are unhappy about anyway, such as the closure of Euston this weekend.
Maybe if they only sold each seat once nutty and then sold any additional tickets as "standing" that would be fair.
It would be impossible to manage. What about failed connections, would you be able to claim back the difference between a seated and standing ticket if you missed your original train? Maybe arguable if the railway is at fault but if you're not on an advance ticket and simply choose to take a later train, would you suck up the difference if you couldn't sit down? The admin alone would see ticket prices soaring. It would mean the end of walk on services and vastly inflate costs. What if you can sit down for part of the journey but not all? If you buy a standing ticket but there is unexpectedly some room on the train (see failed connections above) are you allowed to sit down or not? If not, who on earth is going to be in charge of enforcing that?
I don't disagree that bank holiday travel can be miserable, it's miserable however you travel, not just train. The railways need massive, massive investment but under the current franchise system the train operating companies both have their options limited, and have little financial incentive to do so - and however you feel about this, the fact is they are profit making companies.
A lot of people I speak to at work don't understand the franchising system, and nor did I before I worked in the industry so in no way slating them for not understanding, by the way.