So I fully get how the time travel in this movie works and that they intend to return the stones to the moment they got them, but everything they will have done to retrieve the stones will have totally butterfly effected those timelines. There's no way events play out exactly the same in any of them.
And considering they only had a 1 in 14 million chance of stopping Thanos to begin with, it's very conceivable that they doomed a timeline or two.
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.
The 1 in 14 million chance is only accounting every possibility from THAT point on (from the Titan scene). It doesn't account EVERY possibility ever. So different futures might have different chances.
"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.
The 2012, 2013 and 2014 timelines that they traveled to are permanently distorted. In fact 2014 doesn't even have a Thanos to worry about since he completely disappeared from that universe.
I don't think of it as dooming a timeline. I see it as them entering a timeline that was already fated to become that way. Thats what the actual scientific theory of the multiverse states anyway. That there is a contingency for any possible outcome you can think of that takes place in a separate universe. And if you were to somehow travel back in time to yesterday you are in a different universe but you are playing it out as it was intended to be played out even if you end up going on a rampage destroying the world.