I'm afraid it would be just the same if there were 4 cars already. There would be requests for another track "just to try" or a car with slicks.
At some point, sadly, they have to draw a line and say that's enough, the demo is here to sell the game to people. I think the LFS demo is great. 2 good tracks, 3 of the best cars in the game (imo) and updates as frequently as any licensed racer ever receives them. Plus the countless videos on Youtube and Googlevideo give a good taste of what the licensed content is like.
Unfortunately if you're a demo racer, and you want to try the licensed content, then you'll have to put efforts into getting a license instead of requesting content for free here.