Plot problem I had, the small hand held technology that helped stear each person through time could only move one person through time at a time.
How the hell did Thanos get his entire army, spaceship and weaponry through the time machine with only the controller of a single person?
That bugged me a lot.
This and the fact that Rogers showed back up in this timeline instead of growing old in some other timeline. If this was possible, Nebula would be dead. On the flip side, 2014 Thanos got dusted, so there is a timeline where stark, vision, and black widow should still be alive.
They spend a lot of effort to say that returning the stones would preserve everything in a single timeline...
Keeping in mind it's a comicbook movie... but... for what it's worth, here's a breakdown of the logic:
Step 1: Travel back in time to grab the stones.
Step 2: Once in the present - unsnap everyone back to life
Step 3: Return the stones back to where they belonged in the past - thus preserving the timeline as one.
Cap returns the stones... and then instead of jumping back to the present he just decides to take the long way around and hang out in the past. Once an old man, he just hangs out on a park bench at the correct time they were expecting him to return.
Now, really the only valid complaint you can make is that him staying in the past and presumably marrying Peggy would create an alt universe. The counter to that is this... it's possibly when old lady Peggy talked about marrying her "husband" that Cap saved in the war - she was actually talking about Cap himself... and that the "husband" was always Cap. Just that Cap laid low with an alternate identity and Peggy covered for him.
That's a little trippy, but it fits with the wonky time travel logic of comicbooks. If you think that's bad... try watching CW's Flash or Doctor Who. There's always some ridiculous wibbly wobbly timey wimey nonsense that doesn't really fully hold up if you try to make sense of it.
Now, if they want to... these little timeline inconsistencies can be plotlines of future content. For example, Loki is seen making off with the tesseract... my guess that will be a key part of the Loki TV show they are going to have on Disney+