To be fair, in other countries where this is the norm, reservation only booking online, by ticket vending machine and by app (as well as, of course, in person at a booking office and by telephone) has been the standard for literally decades. Train conductors should also be able to make them for customers to assist e.g. if there are delays and it becomes apparent they won't meet their connection, the guard can just reserve them onto the next available service.
Train Companies in the UK didn't want to do this until now, for reasons best known to them. If they had, people would be totally used to it. Compliance might not be 100%, but honestly it doesn't need to be. These means of getting a reservation pretty much the minimum standard, though.