A "survival of the fittest" approach where those who move fastest & have the sharpest elbows have the best chance of a seat would most likely have much worse outcomes......fancy being the TM‡(resolve) who has to sort that out? Or perhaps you'd like to take your chance & stand from Plymouth to Paddington one Sunday afternoon?
It's precisely the long journeys that are disadvantaged by reservations. It's even worse on Cross Country (no thanks to those Voyagers managing in four cars the same seating capacity as a three-car 175). You can stand from Plymouth to York because of people getting unnecessary online reservations for a random scatter of miscellaneous 20-minute journeys. To a lesser extent, we get it on the London trains with reservations for journeys wholly within England on the Welsh trains and reservations wholly east of Cogload Junction on the South West ones.