In my opinion, going larger than life and having a more grandiose, "franchis-y" story arc to it is what turned it into a franchise. I'm not saying going back to racing cars is necessarily a bad thing, just that it wouldn't have nearly the box office power.
I, for one, had zero interest in the franchise when it was just macho guys racing cars. Once the Rock came in + the story became bigger is when I took notice.
All that to say, different strokes I guess.