That's what I did - played as much as the SP stuff as I wanted, didn't go for gold on everything as frankly, I've got better things to do, then got bored of it. The MP side of things didn't interest me at all, and although I still had maybe 20 odd races untouched - the grind killed the game for me.
If I fancy a McLaren F1, 17 million.... at the time, you could grind maybe 500k in 20-30 minutes, the alternative was spend real money (off you jog!). So yeah, the desire to collect and mod cars, quickly died for me and I sold it on.
A year later, I guess, I picked up another copy, as I quite fancied another crack... I've had the last two 'big' updates, but they've been pretty meh overall. I liked some of the cars they added the E190 Cossie, and recently the Escort Cossie. But track-wise, it's been like 1 new track (I think) and that is the pants one in the snowy mountains.
I occasionally pop on to do the weekly challenges - but only they they net you the 800k in tickets. This game, to me, is more about making money than it is racing.