You are saying that you don't think UPS' automation efforts will reduce the effective workforce by 20% in the next decade? I would strongly disagree with your certainty, and believe if their per-package efforts don't decline by > 20%, they will consider their efforts a failure.
Also, it is possible the union sees no end-game in sight and is just trying to preserve jobs as long as possible. (The robots won't have a union, and if they did, this union would be unlikely to be a part of forming it.)
I think I agree that the UPS union's decision is possibly a poor one, but come on -- it's easy to see why they made it. Making self-interested, short-sighted decisions is literally what humans are best at.
(Plus, it's not like we can pretend to predict the future and better than they can!)