In the third movie of their trilogies, Tony, Steve, and Thor all lost something pivotal to their character. Tony had the surgery to remove his arc reactor; the device that inspired the invention of Iron Man. Steve left the shield behind after fighting with Tony. Hela destroyed Mjölnir during Thor’s first encounter with her. However, they all had something to soften the blow of these losses.
Tony’s arc reactor proved he had a heart. In Civil War, he starts a father/son bond with Peter Parker, who becomes important to Tony’s life, proving furthermore that he has a heart, but doesn’t need the arc reactor to prove it anymore. In IW, his heart broke when he watched Peter fade before his eyes.
Steve’s shield is important to the story of Captain America; it is there to protect him on the battlefield. However, before he went through the Captain America transformation, he had Bucky. Someone to protect him; Bucky put himself between Steve and danger – Bucky was Steve’s shield. Steve let his shield go after fighting Tony because he didn’t need it anymore – he had Bucky back. In IW, when Bucky faded, Steve lost the man who had always fought to protect him.
Mjölnir is there to give Thor control with his powers. Before the first film, he had grown up with Loki by his side; his brother and his best friend. Thor, however, was cocky and had the hammer taken away from him. When he learned his lesson, he used Mjölnir to battle against Loki. In Ragnarok, when Loki finally decided to settle and allow Thor to be his brother again, Thor didn’t need Mjölnir anymore. In IW, when Loki dies, Thor loses the last member of his family and goes on a journey to create Stormbreaker to fill the gap left behind. [Inspired by @fangirl-andproud]
Only, Thor got Mjölnir replaced in the movie, while Tony and Steve’s losses were at the end and basically they didn’t have time to find replacements.
I wonder if A4, with the time gap, will show Tony and Steve both having replacements too. Tony will have another, even better armor in A4 (it was confirmed by someone, I can’t remember rn) and like we’ve heard in hoco, Tony was making Steve a new shield. So when they do lose the things that were pivotal to their arc, maybe again they’ll gain something new (armor, shield, stormbreaker) and when they regain who they’ve lost (Peter, Loki, Bucky), they’ll willingly give it up.
After Thanos is defeated, Thor won’t need Stormbreaker; he only needed something physical to hit Thanos with. Ragnarok made it about clear that he doesn’t need a weapon to wield lightening.
Tony will give up the armor and retire. Maybe get a farm. Away from the superhero life. His arc will be completed with the wedding.
As for Steve, I don’t think it’s physically possible for him to stop fighting, so the only I can come up with is this: he will die and leave the shield to either Sam or Bucky, who will take on the shield and its responsibilities (genuinely hoping it’s Sam) and his arc will end also, along with Tony and Thor’s.
When the holy Trinity retires, a new phase of Marvel will start. Awesome meta op.