There are plenty of glitches to get around the money issue. Duping, gifting, using several PSN accounts and "borrowing" cars to run the Seasonal Events and Bonus Races, A-spec "rubberband grinding", etc.
At some point, it actually gets kind of hard to spend the money fast enough!
And I think that's part of the reason for a cap - if you're at 20m credits, and you still have races you want to run, you have a choice: spend some money, or race for free. Before I'll enter one of the big-payout bonus races, I'll make sure I go buy something nice - a Murciélago, a 458 Italia, etc - just to clear out some room under the cap. A couple times a week, I'll come across something really choice in the UCD or OCD, and spend a few million. It always seems to come back quicker than you'd think... ('Course, I've been known to let my Bobs do some enduros while I sleep or work, which helps. And rubber-band grinding stacks up the coin in quite a hurry, too!)