I would prefer the game to only allow you to use mileage points received on that car to upgrade that car. To encourage people to get used to the car before they make it faster. Currently its meant to make people learn to drive the cars before they max them out just so they can mo the grass on nurburg tourist lobbies.
When people are going to rubber band to get these things in the game, then its effectively wasting electricity and doing harm to the planet, just make everything available from the get go and rid this grinding nonsense. Or better yet, give people the option when they try to install the game to select 'Don't bother installing, just give me all the trophies to show off to my friends list'.
If you have 20 mil credits, all the cars and don't have the mileage points? what the hell have you been doing in the game thus far? except for the money glitches of course.