Let's say that happens, which is not at all a given. What are you going to do when car usage goes down because people are tired of being taxed excessively for them? "Just tax this thing I don't like" is not a viable strategy to fund the things you do like.
Less car usage means less money spent on car related infrastructure and better flow of existing traffic which leads to further economic growth as space freed up can be repurposed for something else which lead to increased tax revenue.
