If you have short trip standing reservations you could keep standing within tolerable levels and maintain flexibility, as a stage to just banning standing on long distance services.
There is after all already an absolute limit on passengers getting on a train so no guarantee of travelling. You just want your convenience to be prioritised over others others who have made the effort to plan their journey.
If you do it and don’t allow fares to go up then it will be easier to justify longer/more trains and harder to politically ignore.
We have the technology for you to switch bookings right up until the train leaves.
OK, so take the WCML. Most of the fast trains are running at maximum length. Virtually every available path is occupied. (Until HS2 is built) there is insufficient track capacity to increase the number of paths, so as soon as the seats are booked/occupied, everybody else needing to travel has to wait until a train arrives that does have seats. At Euston, the station would become too crowded within the hour and need to be closed. That would exclude those with booked seats as well so nobody wins. At intermediate stations, say MK, trains would arrive full, and as so many of them would be pick-up only, nobody would alight so nobody could board. Thus unless demand was very low, an increasing number of passengers would just not be able to travel. Mmm, you don't seem to have thought this through at all!
However, lets continue this
'whatever's good for airlines is OK for the railway as well' and actually propose it is exactly as the airlines have it. If somebody has an open ticket that is valid on that train and is refused boarding, (as in 'bumped' airline style) then they are entitled to compensation and if no promise of completing a journey that would otherwise be complete that day, also provided with overnight accommodation. So your £60 off peak single ticket from Euston to Birmingham New Street would entitle you to £100 for delay and if the TOC couldn't find you and 50 other passengers (per overcrowded train) a seat on the same day, there would be a £100 hotel room coming your way as well. Onn a bad day, that would cost tens, maybe hundreds of thousands of pounds compensation. Forget what passengers might say, do you think that the TOCs would still want that?