Starfleet saying Picard was wrong doesn't make it wrong. Picard did the right thing in not making a snap judgement to commit genocide.
Also, why would millions of drones find themselves free? What if it killed every single one of the trillion drones in the galaxy?
Who knows? Maybe Janeway destroying the Borg complex freed one trillion drones that would have been dead had Picard proceeded.
edit: In reading the synopsis for "I, Borg"...I am reminded that it was Picard's idea.