Well no - if I thought I was likely to be in my reserved seat with unreserved people's bums in my face and blocking my way to the doors/loo and any hope of the trolley then I will just drive. Its bad enough on short distance commuter trains, unacceptable on intercity
Only if the bag has a sweaty bottom IIRC (or was that another train type?).
You would hope that if you sat in the seat before/whilst the train left your departure station then there would be a long delay before it assumed you had moved seat/gone to the loo. Would there be a downside of an app based reservation system letting you change your seat reservation whilst on the train (to get away from a smelly/large/loud person or to get a window for example), and even having a 'I'm popping to the loo/shop' button to assuage the worries? It could even let you hit 'need the loo' and tell you which way to go to find a working/free one!
Not a bum! Whatever will we do?
I don't think one passenger's preference to not see a bum outweighs say, the need for someone to travel full stop. But however you look at it, the problem with our railway is capacity. On some routes (cross country and emr) trains are way too short. On others too infrequent. Fixing that is the answer as it will ease overcrowding for everyone and all these problems would fade away without bizarre ticket restrictions, seat reservations and wierd apps. Hopefully hs2 will help with this in some ways and on some routes.
Compulsory reservations also massively underuse the capacity available. I often get emr trains between derby and London. I get on and the train is packed standing. Everyone empties out at Leicester and I sit down, along with many others. If it was mandatory reservations I'd simply never ever get to Leicester. Then, that same train would do the bulk of the rest of the journey with loads of empty seats. I'd be stuck at home. Pretty daft.
I any case, all of the above would be sorted out to a large extent if the emr trains had been ordered with lots more carriages. But the country is run on a spreadsheet by people who are removed from reality, so the HSTs were replaced by trains half the capacity. Forgive me for not trusting that HS2 won't be similarly underfunded when it comes to trains.