Yo. I figured it out. Hear me out. Here is my theory. Sorta.
If this movie is 5 years in the future. They probably were just stunned on what had happen. They can’t possibly do anything cause it’s beyond everyone power to search for Thanos throughout the cosmos.
Cutting back into the time line of the movie...
After the initial battle and defeat. Nebula assistes Tony and send him back to Earth.
The Avengers are United. Defeated. Blamed for what has happened. Everyone has lost their love ones. They are marked as villians are deemed wanted.
Wakanda is in shambles, a new King steps up and it is Mbaku. Shuri assist on the building up of Wakanda and rebuilds Vision.
Hawkeye comes out of retirement due to him losing part of his family. Ant-man unites with Nomad introducing Captain Marvel.
Ant-man and Captain Marvel figure out a way to time jump with the quantum realm. The only person to survive the time jump will be Captain Marvel and possibly vision and ant man.
There is gonna be a time jump back to the battle of NYC in Avengers 1.
Hence the footage of them in their older suits with only the Original Avengers left. The past avengers are introduced to Captain Marvel, Antman and vision. They send them a message stating that earth is doom, and it starts at this battle. So instead of closing the portal, There gonna fly into the portal(the same one Tony goes into to save the Earth from the Chitarui forces) and go directly to Thanos ship prior to him having the gauntlets.
Thanos is surprised by the avengers but is ready to do battle. The battle is more balance since he doesn’t have the gauntlet and Thor doesn’t have stormbringer. Thanos does work still on the majority of the group. Captain marvel joins the fight and puts a molly woop and then when the wipe his ***. Vision destroys Thanos and They use the Tesseract to bring everyone back to Earth. All is restored in that timeline.
I’m not really a fan on how it brings everyone back. But if they went into the past. Then they are in a different time line and are affecting that timeline. Not everyone makes back to earth. Captain America and potentially Tony stark dies.
Thor and Hulk gets blasted into space randomly (world war hulk).
The sequence of changing events resets everything that happens after Avengers 1.
I can’t really figure out how they introduce everyone that died. Maybe in the current time line it shows everyone being phased back, gamora comes from the soul world. And those died(Loki, etc) are actually dead for good.