Wow six pages of nasty snide little thieves coming up with increasingly rude and abrasive reasons why it's fine to steal something that belongs to someone else.
Obviously nicking something that belongs to someone else is rude. Sit in a reserved seat only if it's clear it's unused (eg London-Edinburgh but you left London 5 mins ago), if the seat reservation is Leeds-Edinburgh then it's fine to sit there from London to Leeds to either get up when the train reaches Leeds (sit back down only if the train leaves without the rightful owner of the seat turning up), or at the very least look around and be ready to spring up if someone approaches and is clearly looking for that seat. There's a big difference between that, and people who spread all their stuff around and put earpods in, staring out the window, pretending not to notice people asking for their seat.
And sneering and saying "well the onus is on you to ask for your stolen seat back boo hoo if you find confronting thieves difficult" - jesus, is there a full moon or something?
Plenty of people are neurodivergent, deaf, have mental illness, have PTSD, or don't speak English. Or have just had bad experiences on trains and are afraid of potential confrontation. There are lots of reasons why someone can't physically ask for their seat.
And unfortunately you can't predict how people will react to being told they're in someone's seat. Some people are lovely and apologetic and move, some get verbally abusive, some just flatly refuse to move. There have been tons of threads here where the OP as asked someone to move from a reserved seat and the other person has become abusive or belligerent, even one thread where the seat-stealer spent the rest of the journey heckling and yelling about what a C word the OP was. (Probably mental illness at play there.) I've even had one man lie to me that he was travelling with another person who was in the toilet just to prevent me from sitting in the empty seat next to him because he didn't want a disabled person near him. Look at this very thread, posters are saying it's "pathetic" to want to sit in your reserved seat. How selfish and rude!
The point is, it's not "no big deal just tell them they're in your seat and ask them to move!" when you have no idea if the person is going to react by apologising and moving, or by punching you in the face. Since Covid the amount of violence and hostility and just generally the amount of feral, antisocial behaviour in this country has shot through the roof.
I'm disabled and can't stand without pain, but I also can't speak brilliantly due to aphasia after a stroke a couple of years ago. But according to Mumsnet stroke survivors should just be forced to stand because fuck 'em, right? Fuck anyone who can't fight for themselves.
The "I've got mine so fuck everyone else" mentality here is feral.