I agree that the demo content is plenty to keep people interested. In fact, I think it's amazing to see so many regular demo servers available at all.
Sorry, that should be past tense since I got my S2 license
The only way I can see to prolong the demo's life without adding or removing content is to force players to complete the tests with certain results to make the cars available.
That would ensure that they knew the basics, although I'm not sure how happy people would be about the restriction.
Here's an idea for a new test that comes after all the others:
T1 Test
A series of race starts designed to teach you how to not crash into other players at T1. Depending on the severity of your crash, you receive a different insult, ranging from the mild side-collision "Ouch!" to the 17 car pile-up "WTF u f*cking doing u utter f*cking moron! Get the f*ck off our f*cking server before I come and beat you to f*cking death with your own f*cking arm!!!!!!!!"