Watch 'I, Borg'.
When he made his decision, Picard was ABSOLUTELY CERTAIN that the paradox will work. In his subjective decision-making forum, that the paradox will work was a certainty.
And this was true for the rest of the crew. And given that many from the crew were more than qualified to make accurate judgments on this kind of matters, the paradox had a VERY HIGH CHANCE of working, of dismantling the collective mind.