Old Cap sitting around chilling when he knows the time, date, and place that Bucky is killing Tony's parents would be terrible. Especially after everything in Civil War.
"But he can't change the past"
Well he has changed it by going back and living there so it's a moot point.
If he could change the past he wouldn't be saving Tony Prime's parents.
The MCU model of time travel is just lame. The other timelines exist to have Infinity Gems when the prime timeline doesn't, and get horribly messed up when the prime timeline needs them. But if they can't change the past, putting them back wouldn't erase the damage, only lessen it.
Also the prime timeline still doesn't have stones anymore. Maybe they might fix it by Doctor Strange 2, but until then everyone's still doomed.
Banner/Ancient One's dialogue is logically sound, but "Cap went into an alternate universe and marries an alternate Peggy and time travels back" doesn't roll off the tongue very well. It's just not a good plot point.
I'll give them credit for one thing though: they didn't erase the snap. They had the heroes deal with the fallout and save lives where they could.