Because it is the 1 out of over 14 million futures in which the Avengers won, you would expect that all of the surviving named characters would play a role that no one else could do, so they were integral to the plot in one way or another.
Tony for example:
had the biggest part to play, from perfecting the time travel to being the one to use the stones to wipe Thanos and his forces out.
But even characters with less important roles still mattered.
Rescued Tony from space.
Was the one who figured out Thanos' location at the start of the movie, allowing them to go after him.
General knowledge about Thanos, the location of the Soul and Power stone, and her second(?) self was important in getting Thanos and his army to the battle that they lost.
Died to get the Soul stone, and organised everyone and kept them together for the five years.
Provided the basis of time travel.
And so on, everyone seemed to play a specific role that no one else could really have done, however I cannot think of anything in particular that the following character did, which another character could have done themselves:
What did they do that means they needed to survive the snap?