Avengers: Endgame - Everything That Changes If The Heroes Steal The Infinity Stones From The Past
1. The Decimation Is Averted (Temporarily)
The obvious reason for the Avengers ever entertaining the idea of going back in time in the first place is to undo the Decimation that rocked the Universe. If you take the Stones from the timeline where Thanos knows they are, he cannot get them, he cannot mount them to his Gauntlet and he cannot snap half of existence away. He cannot "balance the universe" and in his eyes it will still be destroyed.
Crucially though, that doesn't change Thanos' agenda. He would still be a murderous monster seeking to rebalance the universe the same way, it would just take him longer. If he didn't have the Stones, he'd find another way, so part of "beating him" in Endgame will be reframing his thinking.
And on top of that, Thanos will chase the Avengers in time if they don't steal the Time Stone first (which they won't, because dramatic armour protects it), so halting the Decimation without stopping him from wanting to steal the Stones right back and continue on would only be a temporary measure. Again, he needs to be persuaded to drop his bone.
In all honesty, there are also more ramifications that we don't know for certain yet because of the rumoured time jump. If Endgame takes place four years after Infinity War, that's four years of history wiped out instantly. That's potential children, relationships, hero dynamics... And of course, there are things like the unknown fates of Clint Barton's family, of Valkyrie and Shuri that count as possible things that would be changed. These are not small things.
The conclusion here is that Marvel are making a brave and dangerous choice to bring time travel into things. They're making a rod for their own backs and making it very, very difficult to pinpoint a place in the timeline where the Stones can be removed safely in order to not cause too much damage to the timeline.
Want to know where and when? Keep tuned to WhatCulture, there's an answer to that too.
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