Surely one could construct a similar argument to say that every seat should be unreserved and it’s a case of first come first served.
I don’t need to book a space for my car to use a road, and the train should be the same. It’s simply not always practicable to plan, and nor should it be, so why should those paying the most for this flexibility then get hit a second time by finding most seats reserved? Of course if trains had enough capacity and weren’t stuffed full of cheap advance ticket holders...
The converse argument is that those on Advance tickets HAVE to use that train - if it is busy, they have no choice but to travel.
Whereas a holder of a flexible ticket can wait. They can decide a seat is important to them and travel at a non-peak time- a 500-reservations train is not representative of the norm on LNER - or they can choose to use the busiest train and face the possibility of standing.