At the end of The Truman Show, Truman finally discovers
it was all fake after all. It was just a set. He finds the sky-painted far edge of the Omnicanecosphere, confirming his suspicions and shredding any final lingering doubts that it was just in his mind (not helped by Christof cuing the sun way too early).
What were Christof’s hopes or what was his motive and alternate plan in trying to get Truman to stay by talking to him over the intercom? Why bother? It was over. Truman
knew it was all a lie at that point. That his life up to that point had meant nothing, but entertainment for millions and that he'd had almost no privacy or genuine uncontrolled experience of anything.
Even if Christof managed to convince Truman to stay, Truman would NEVER look at Sea Haven or his life the same way again and he would cease being the “good ole natural” Truman that everyone in the audience loved, thus ratings would have slipped. The only logical thing Christof could have done was throw in the towel and
let Truman leave through the "sky" exit. As soon as Truman found the exit Christof should have known it was over and instead acquiesced to the producers to cancel the show, ceasing transmission.