Choice is king. If someone wants to risk standing to be on the train of their choice, let them, but advertise that reservations are recommended on certain trains. Making passengers plan their day around the railways is not progression, its inconvenience. Personally I mostly book on LNER and have rarely bought walk ups with them, but on Avanti for example, the walk up fares are more reasonable since more of the route has competition.
It would get messier when a booked 10 car 80x is short formed as a 5 car and many reservations have to be revised. Then what if a passenger forgets to do so, is the ticket then not valid? The UK's rail system is unique and will never be like the continent.