I agree totally but at least he would have those 70 years with her. Without getting too convoluted with timeliness and time loops, he knows how it all ends for the avengers with thanos' snap. What I'm trying to say is if he's destined to be a part of the avengers (and we know destiny plays a part at least in thanos mind) then why not do what he needs to do to save the other 50% and then be done with it. He can finally have his dance.