I came back to this thread to say that I was wrong. At first it seemed it would be perfectly plausible and easy to progress through the game normally -- it really does seem that way at the beginning -- but that turned out to not be the case as I played for longer and longer.
Two days in and too much time spent playing the game, and I only had 90% on the first set of races, and about 70% on another. Three of my "best" cars of the five I'd earned were fully upgraded, and there was still no getting first place in some of these races.
The last straw was when I could swear some people are cheating at this game. I'm racing against a car that is lesser than mine, or the same model as mine when I have the car with all upgrades, and this same player would - every race - zoom past me and go off in the distance ahead.
It truly would have taken me months and months of hours of playing a day to earn everything -- and this wouldn't be due to being terrible at the game.
There is no option for skill and practice to be the earner of progress in this game, unless a person wants to spend an alarming amount of time -- seriously, hours and hours a day, for months -- to play all the way through.
Thank you very much, unibility, for that hack! It worked for me using iExplorer and my device is not jailbroken. It really feels like a weight has been lifted.