In some places you don't really get a choice. Try boarding a Cambrian Coast train at Machynlleth. Even if you position yourself right by where the doors should be (there aren't any stop markers!!) and stand as close as you can, you're more or less guaranteed to get people shamelessly slipping into the gap in front of you in order to nab the table seats.
Unfortunately this shuffle seems to be unavoidable - the first time I went, I boarded at Birmingham expecting to go to Pwllheli but we lost some carriages at Shrewsbury and the Pwllheli portion came from the sidings at Machynlleth. The second time, it was announced halfway through that all four coaches would be going to Aberystwyth due to the number of passengers waiting at Aberystwyth and so it was off at Machynlleth for the train from the sidings again.
The first time at Machynlleth I clocked what was happening sharpish and strategically positioned my arm so nobody short enough to slip under it was able to. I took a seaview table seat on one and one other passenger who got on at a later station was speaking about me pejoratively in Welsh as if I couldn't understand what she was saying. She was saying I was selfish for sitting there. There's nothing stopping anyone occupying the other three seats. I actually don't mind sitting in an airline seat but my laptop wanted charging and those trains only have three pin sockets at tables - the airline window seats just have USB chargers. The second time I got it wrong with my standing position and didn't get a seat of choice for a while.
Similarly at Mallaig, I clocked how many people were buzzing around the station and I was on the platform stood by the door 40 minutes before departure. Once the doors were released I got on and got the Glenfinnan Viaduct view table seat above the engine I was after. One woman who got on was trying to get my attention, presumably to get me out of that seat but I had my headphones on and just stared out of the window at the Jacobite. Eventually her partner called her back to a table on the other side. She made an utterance then stomped off in a huff. I could hear her arguing with him about it. I shared the table with a family of three, no big deal. It was a four carriage train that eventually got packed end to end and I ended up giving up my seat for a pensioner.
Clearly people have no issue taking seats that are preferable to them and it's doing a lot to assume what other people want to do. When I'm travelling with someone, if we can't get a table it's preferable to sit at opposite aisle airline seats than in a seat pair. In a group of 4+ it is preferable to be together at a table but if not, no big deal. That said, people do offer to move when they see we're a group on a leisure trip.