They owned the big houses and lots of land before the show started. Their success is why the show started.
Many of them have said many times in different interviews that the show and popularity will not last. They have said they will make the most of it while it does, and that includes using it as a platform to get out the word of God.
I think they have succeeded in doing that. If they stay on, it will be under A&E's rules of censorship and I don't believe the Robertson's will compromise their faith just to stay on the air. At least I hope they don't.
Like you said, they don't need the money since they were already successful...