I'm not sure if I buy into the whole forcing someone to move up. I think its kind of lameo for people to run stock forever, but thats their perogative. As Brandon pointed out, most people in stock are 50% off drivers who have jobs and live outside of RC. Even if they are uber masters of stock its still a form of recreation and if thats how they want to have fun so be it.
Stock at a big race is competitive and having guys that are super committed makes everyone raise their game. Do I want to move up to mod one day, sure I hope some day I can, but there are guys who really just want to be really good at stock and stay there forever. It makes them happy.
Finally, although the "lifers" are always a factor its not like they win every race. They provide a stable group of high level drivers for the up and comers to shoot for and often beat. They are also a good resource for drivers struggling like myself to improve their big race game. The "lifers" may take away your $5.99 trophy, but they make you a better racer in the process.