Gomers can fix a trans. Dealers can only replace them. Most of the best technicians go to work for themselves. Better pay, and they can refuse jobs that they hate doing. The remainder, actually like being at a dealer. But they ain't the majority.
99% of failed transmissions at a dealer were just a small electric part, or a leaky pump. The trans was fine. They just swap the whole thing, that's all they do.
Like J said, you ain't going to kill one. But you might be able to get a small control part to fail, and they'll tell you the whole thing is bad. It's a core shops dream. People are sending back perfectly good transmissions as cores.