When he made his decision Picard WAS CONVINCED THAT THE PARADOX WOULD HAVE WORKED, and he KNEW THAT THE BORG WILL CONTINUE KILLING AND ASSIMILATING TRILLONS, that they are doing this even as he deliberates.
Picard INTENDED to condemn to death and assimilation TRILLIONS!
There was ALREADY evidence against this...the fact the Borg existed after Data made them sleep and destroyed their ship in BOBW would leave one to believe the Borg would not commit the fatal (and stupid--for any advanced race) mistake of having all their eggs in one basket..so to speak.
All this 'evidence' was known to Picard/Data/etc. And they also had an incomparably better understanding of borg tech than our shalllow information.
And yet, in 'I, Borg', Picard&co HAD NO DOUBT the paradox would have worked!
In his subjective forum, Picard INTENDED
to condemn to death and assimilation TRILLIONS, despite being certain that he can save them! Essentially, Picard intended to be accessory to GENOCIDE on an enormos scale!
All that so that he won't have to 'use' a single person, Hugh, to destroy his 'culture' - as I said, the episode could even be interpreted in the sense that the paradox would only shut down the hive mind, liberating the drones.
All that so Picard can delude himself, when he watches himself in the mirror, that he always took the perfect moral choice, choosing to ignore the blood of uncountably many that stains his hands, his conscience.