"They return the stones because without those stones those timelines would be chaos (according to the Ancient One). And yes, those timelines would definitely have different futures."
That's actually my point though. The simple act of going back in time, altering events, and borrowing the stones is already going to spin those realities into an entirely different timeline and events will play out differently. Therefore, why even bother with the damned time heist? You may as well just go back in time, explain the situation to your past selves, and borrow the stones. Honestly, you'd be doing them a favor by telling them about the shittier situations they will face in the future. If anything, Cap screwed over his past self by making Hydra think he knows about them and making him a major threat.
Stealing the stones covertly served no purpose.
You may as well just go back in time, explain the situation to your past selves,
Which was the plot of Xmen DOFP
Don't get me wrong. I loved the movie the way it is, but that's kind of a large plot hole.