If its about affordability, then thats something else. Because someone is on say £50k a year it doesn't mean they can suddenly rustle up and pay a £5k fine at the drop of a hat. People on high salaries already fund the roads more than those on lower salaries. I think when it comes to crime, its a fixed rate and then a licence removal - I cannot see why authorities have to wait for four convictions before even looking at a licence removal.
I think you are including the uber rich who have tonnes of disposable income, sure footballers and the like, thats a whole different ball game.
If the government can show some data that shows higher earners are flouting speeding laws much more than lower earners then sure I can go with the argument, my guess is that they don't have that. It wont stop lower earners speeding, it might stop higher earners speed but if lower earners are say 80% of the problem then the government are going to fail in reducing speed related incidents with this measure.