It’s been a long time since I’ve seen 12 Monkeys. My impression has always been that the end was meant to be tragic. That they were so close to being able to figure it out (the one person having actually been there at the time), but ultimately they never did, and never prevented it because it always happened. The scientists in the future are so focused on the 12 Monkeys group that the person that actually released the virus sits comfortably in their blind spot.