This is an exaggeration. Even if the parking charge was £5, I doubt there are that many people who would be unable to walk becasue they can't afford to park their car. If there are, they can't afford a car in the first place, and they have far worse financial problems to worry about. If anyone is put off by a small parking fee, they can't be that enthused about walking in the first place, or they can find places to walk where there are laybys that are free to park in.
I've heard this sort of argument in my local town of Horsham when they introduced Sunday parking charges. So much whinging about it, trade will drop off etc etc etc. The end result is that the town is rammed full more than it ever was on the weekends. People just don't want to pay for things that they have got used to having for free, then try (badly) to come up with reasons why paying will cause all sorts of slippery slope effects, that is all it is.
Before anyone decides to throw out the obvious strawman, no I am not saying that any charge, no matter how high, is justifyable.