It seems only 4% of the World oil/gas production is used for plastics.
Two things to your post though. Yes, making things out of oil is probably better than burning it. But don’t mistake how much oil is used in the side-processes.
We will burn the oil one way or another. Until there is a better source of energy with better safety and density than hydrocarbons, we’ll use it all. If not in cars, in power plants, if not there in military, if not there in 3rd world, etc.
I'm sure composites make electric cars better, but I don't think electric cars are impossible without composites.