I know this question has plenty of answers, but here's another, from a different perspective.
Thematically it had to be Tony, every avenger movie so far had built to this.
In the Avengers Assemble when Cap and Tony are facing off Cap says "You'd never make the sacrifice play, fall on a wire to save your friends".
However later in the movie Tony makes the sacrifice to
take the nuke on a one way trip to Chitauri-ville and only luck has him survive.
In Age of Ultron it was Tonys
chest blast that blew up Ultrons improvised meteor, but it blew out his suits power, he only survived because Thor rescued him.
In Infinity War, he's defeated
- and about to be killed - by Thanos, he'd have died except Dr Strange saves him.
In every Avenger movie Tony shows he is willing to die to save others, it was always going to be
Tony that would die to defeat Thanos.