Depends. If you enjoy buying cars, tuning and testing them, then move on to another car... the game doesn't "lavish" money on me IMO. I've spent a lot of time already doing the endurance races over and over again, just so i have enough money to buy and tune more cars.
Little of that hard earned money (spent many gaming hours, or let's say... hours of my precious free time to earn it) has gone into decals from the storefront, and i'm more than happy it didn't cost a fortune aswell, like buying and tuning all the cars did.
What you also forget is that a good decal will get purchased a few hundred times probably, so it's not just 10'000 credits you earn for the hard work, more like a few hundred thousand.
And if somebody really thinks their design is worth 999'999'999 credits, he/she can still sell it on a car in the auction house (same goes for tunes i think) for whatever money they want for it.
The 10k cap is one of the best things ever in FM3 IMO.