As we see in Avengers: Endgame, Steve Rogers lived his life with Peggy Carter by going back in time. When he returned, he was old. He passed on his shield to Samuel Wilson, the falcon, declaring him as the new Captain America.
Even in the comics, the role of Steve Rogers as Captain America is replaced by Samuel Wilson. Sam Wilson was first introduced in The Winter Soldier. There he helped Captain America and Black Widow in fighting against HYDRA.
So, what happens when the world needs Captain America to save it?
1. Samuel Wilson as Captain America:
Now, as mentioned above, Sam is the Falcon and now, also the new Captain America. As Falcon, Sam used mechanical wings to fly and in the comics, he had limited telepathic and empathetic control over birds. Steve Rogers chose him to be his successor.
In the Comic, he becomes the leads the Avengers against enemies. Maybe we could see the same in the future.
But Steve Rogers is Captain America mostly because of the super soldier serum. Sam doesn’t have the super soldier serum, he’s just got his wings and goodwill in the MCU. It would be interesting to see how the Falcon stands up to the role of Captain America in the future Marvel movies.
2. Time-travel:
Since time travel has now been introduced in the MCU. There are two Captain America in the same timeline because Captain decided to live out his life with Peggy. To not wreck the timeline wholly, the Captain should limit himself to a few people and not take part in any battle.
If we ever need Captain in the future MCU problems, we can bring Captain America from the past (the one where he is living the Tony talked about :’) ), and once the work is done, send him back from where he was picked.