Wait a minute, what other mode of transport is there other car or local bus? Aeroplane and long distance coach maybe.To be fair the TPE example should never have arisen and was a rather extreme one. But yes, if the train is full, perhaps people should change their plans or mode of transport, as they would for any other mode bar car and local bus, in order that everyone might enjoy a comfortable journey.
If we take your logic to it's natural conclusion, you're saying people should be barred from going onto the motorway at busy times, to reduce the risk of a traffic jam?
And those guys who need to get the train home? Tough tittie, you'll just have to camp out on the station until, well I don't know when, but until there's a train going to your destination that has less than 100% of seats occupied.
Just last week I went to London. Train in medium loaded. Train back full and standing. You're saying it would be reasonable for me to kick my heels in London for several hours, or "change my mode of transport"? To what would I change it? Get an Uber home? Hire a car? Would those be better for me and the environment than just standing until we got to Chelmsford where I got a seat? Would it have made any measurable difference to my fellow passengers?
Can't you see this policy is absurd in the real world?