Obviously this is a DfT led cost-cutting move above all else.
Putting that aside, genuinely what would charging for reservations offer compared to simply giving advance ticket holders a tickbox to opt out of a reservation? Plus not offering them at all for sub-20 min trips.
Helps with the empty reserved seat problem while not taxing people who do genuinely just want a guaranteed seat.
Of course many people will still opt for a reservation and may not use it in practice, and a £1-2 charge may encourage those people to actually sit in the seat they reserved. But if this is only a minority of people, then it's not important and shouldn't require having all others pay extra. If it's more than a minority, i.e. a regular occurence on most services, then at that point you may as well get rid of reservations altogether to solve the musical chairs game. After all, if there's capacity on the train for the reservation holder to sit elsewhere, then most people are unlikely to have issues with finding a seat if reservations were to be abolished.
Then again, if TPE were to run longer trains on the WCML services and eliminate 3-car 185 working altogether, this wouldn't really be a problem. The upcoming BEMU replacement stock only being 5-cars wasn't a great move unless they're committed to running 10-cars at peak times.