You seem to have a bunch of holes in your theory, man.
1. Consider the community:
Whether you like it or not, there are going to be punters online. What do punters tend to do? Wreck your car. When you're driving cleanly and get blindsided by some dick because he feels like having a good time, do you really want to foot the bill?
2. There's gonna be accidents:
What if you're driving online and the person in front of you accidentally slams on his brakes because he misjudged a turn. Yeah, you're supposed to race your time, not the person in front of you, but not everyone follows those rules. In this scenario, both of you made simple mistakes, yet you're going to be in the hole however many credits because you lost track of things for a second.
3. Online can be laggy:
If GT5P is anything to go by, there are going to be moments where a car that's 4 lengths behind you is magically transported right in front of you thanks to lag. Heck, maybe he'll be transported inside you. Now, if there's a definitive damage in all online races, this race was just ended. Lag is unpredictable, and you can't just tell me that everyone is going to have a perfect connection all around the world.
So, if you just read this, and your first reaction is going to be telling me to go play Forza, then maybe you should go back up to the beginning of this post and read it again.