Oh good God, just make room in your thinking for variations amongst people. It's true that it should have been more difficult for Picard to tear himself away. I think they would have needed more screen time to build that impression up, to show him becoming more deeply immersed in the fantasy, then more time to show his mental struggle against it. There goes all the screen time... Anyway, Guinan had the choice made for her. She was plucked out in the early stages. She knows what that part of it's like. She reported the absolute truth of it to Picard, the Nexus was that perfect happiness you'd seemingly have to be crazy to say no to.
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Picard had more time to wrestle with it. If Guinan had had that time, maybe she would have found her way out too. Maybe not. The point was, it's HARD. It would severely test any person dumped into it.
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People nitpick this film to death, as if lack of continuity errors makes a great film. Talk about missing the forest for the trees. The concept is what's important, what if Heaven was a place you could get to by murder and trickery, would you?