I would say that anyone that loves racing will simply move up and would not quit. Especially after spending thousnads on these toys. If they do then thats their loss really. The only stipulation I would put on that would be that if the class above had no racers to race against. Then you have to let them race where there are people to race against.
Also depends a little how good they are. I mean if they win every week by large margins then move up, they are hurting the sport more than anything. A new guy comes in tries everything he has but gets beat by 5 laps every race.. Now he quits cause he hates that he isnt as fast. Which do you think is more likely to quit, or a worse loss to the sport? If the "champ" is, has been in really close races all season and only just won the points race for the championship, but lost several races in teh season and so on, maybe several drivers could have won it but this happend or that and they didnt. Then maybe the line is less clear cut and you could ask him/her to move up to a tougher class but not really insist on it.
isnt this what a novice class is for though??