If you want to understand the theory properly, please read our Time Travel Theory Explanation, because it’s going to be a bit scientific. So important things said, Let’s Begin.
Ego’s plan was to kill everyone in the Universe except himself alone. He wanted to be the sole ruler of the universe. But somehow Gaurdians of the Galaxy stopped him from doing it. They destroyed his brain which was in his selfmade planet, called Ego The Living Planet.
In Avengers EndGame, the Avengers traveled back in time where they had made multiple Alternate timelines. By taking away each stone they created one more timeline which was different then the main, MCU. So the events of Gaurdians of the Galaxy Vol. 2 took place in 2013. And Nebula brought Thanos, And his Army to 2023,The events of EndGame.
So, it means there are no Gaurdians in 2013 as Gamora and Nebula are here and they don’t form a team. And if they don’t form a team, Ego takes Star Lords with him and does whatever he wants and gains access to his powers. And Destroys the whole universe. But only in alternate Timeline. Which doesn’t affect the Main Timeline.
This was just a fan speculation of alternate Timeline by a Reddit user u/Ljgor. And do you think does this makes sense that Ego killed everyone in MCU Because Avengers are way far from traveling to Space.
I think alternate Timeline does exist in Marvel Cinematic Universe because, Thanos and his Army are dead in 2023,which means that they are missing in 2013. And if they’re dead how can they put them back or send them back to 2013?