So as nice of an idea as this would be, it would be heavily impactful to server performance.
Since the base game vehicles are involved in the game itself, we dont need to worry about file size with them in terms of civilian vehicles.
For us to replace each of them with an addon vehicle, it would mean that we then do have to worry about an inflation of file size.
More space taken up on the server = Lower Performance in dense player areas. If we were to do this, it would be heavily detrimental to the experience, therefore is not something we're looking into doing at the moment.