He would have won about another $10,000 and the next time he was on he would not be given the answers and his opponent would be fulled fed every answer. His questions would probably be more tricky and aimed at places the producers felt confident he was not as knowledgeable in. He would lose and be completely out. He was hoping for that panel show and money coming in from that (although it would never match what he was making on the game show). I guess he also just wanted to stay on TV. Giving the wrong answer seemed like the only shot he had at that.

In one of the flashback episodes, one of the former contestants did exactly that: gave the right answer when he was supposed to give a wrong one. Jack Barry does a double take as he start to say "Oh sorry" and then realizes he gave the right answer.
As other posters mentioned, if someone was uncooperative, they would either have stop feeding answers or else slipped in a totally new question.

I think they should have just stopped cheating and ultimately let him lose on a hard question; one he actually did not know. That's just in real life, though. I think the film is great drama the way it is.
"I will not go down in history as the greatest mass-murderer since Adolf Hitler!" - Merkin Muffley

I really think they did it to humiliate him. They didn't think much of him to begin with, he was getting difficult. They did it just to be mean.

But why kill Stempel with an "easy" question?
"I will not go down in history as the greatest mass-murderer since Adolf Hitler!" - Merkin Muffley

He was trading winning on 21 for the panel show and possible future fame. As you said, the promised they made were false, but he didn't know that at the time he took the dive.

From his perspective, he had nothing to gain from answering correctly. If he betrays them, they would have found a way to get him off the show short after (stop feeding him answers). Therefore, he knew that his only shot at a possible future was to play ball.
Ultimately, all of the promises they made him were false, so he was damned if he did, damned if he didn't.

You're right: He would've never been accepted in another quiz show. But it's worse than it sounds. He desperately needed the money. This becomes more obvious after he "loses".

They would just lie and make him look bad just like they did when they played that fixed tape in the office.