I don’t think Steve went back in time because Peggy was a prize he deserved, the happy ending the franchise owed him. I think Steve went back in time because Tony is the center of the MCU and they needed to give Steve a happy ending that would still be less heroic and less satisfying than Tony’s. Steve, like Tony, was inevitably exiting the MCU - Evans extended his contract for just the one movie - but they weren’t willing to give him a heroic sendoff, as they had decided to do for Tony. Steve is more beloved and more heroic than Tony, and it would be impossible to not have Steve outshine him if he did.
It could in part be creator contrariness where they refuse to do something simply because it’s what fans expect, predict or desire. Steve’s death is famous in the comic Civil War arc; this has been a major fan theory for several years. But as a fan theory it was directly related to how many people love Steve and feel he, not Tony, is the center of the MCU. He did not get this ending because the franchise has been very firm on keeping Tony in that role. Steve has been allowed to be heroic in a frankly patronizing way - one non-game-changing moment of wielding Mjölnir to pacifiy fans, one highly quippy “Avengers… assemble;” Steve is written as a mascot, not a protagonist.
Frankly even the decision to show Steve in old-age makeup at the end of the film feels like a calculated attempt to make Steve feel less virile than Tony - now he’s a revered and respected and beloved grandfather, not a beefcake cut down tragically in the prime of his life, like Tony, leaving grieving loved ones and the potent grief of what he might have been.
(Contrast this with Natasha’s death, which takes place because her life is apparently so empty as a result of her lack of family - this is hammered in throughout the film; they took the infertility plot of AOU and drove it into subtext - Natasha mentioning she never even knew her father’s name before now, giving up her life because Clint still has children they want to resurrect, Steve saying they were her only family: it’s very much the same misogynistic tale. Natasha is framed as giving her life because she’s more valuable to others as this sacrifice than she is alive. Tony is framed as someone who had so much to live for and will be remembered forever at this peak of his existence, giving his life for the world.)
While the reduction of Peggy to prop is still galling, in no way is the ending written to honor Steve. Proof: Sam and Bucky take it extremely well. These are Steve satellite characters, not even written in such a way where we can understand if they even have lives outside of him. Yet both of them take it completely in stride when they realize he has abandoned them and is now at the end of his life - will soon die of old age and there is nothing either of them can do about it - will never be there to save their asses in a battle again, will never be their rock to lean on, will never need their advice or companionship. Bucky, in particular, has had as tough a time as Steve readjusting to the new era, and harder, with his HYDRA-induced traumas, yet he doesn’t even get Steve’s parting words.
Superficially the narrative respects him. He is a symbol, but he not a force of his own, does not inspire any particular passion or devotion that would lead to any real objection to his actions in the finale. A character who was written to matter would never actually be received by this. And that’s even before you account for Natasha.
Since Avengers, where they instantly sparked off one another, Natasha’s arc has been tied to Steve’s. Though she was introduced as a rather antagonistic foil to Tony, Tony’s story and characterization have never particularly had room for her, except to remain in the role of opposition. But she and Steve gel differently, and Steve’s story and characterization had enough room for her to have an entire separate character arc. Steve’s moral values and existential struggles brought hers to the fore. He made her a stronger character, and his values system being powerful enough to produce an entire growth arc in Natasha only reinforced Steve as a protagonist. And all of this is why, after The Winter Soldier and its impact, both Steve and Natasha related to Steve got massively scaled down (she has as much time with Tony as she has with Steve in Civil War, and in Age of Ultron was paired off with one of Tony’s satellite characters - keeping her far away from Steve, and having her renege on the new values she had learned in her plot with him). Compared to how they naturally fell into sync in Avengers, and considering they had all of that growth together in Winter Soldier, it becomes more obvious that the films have actively made an effort to separate Steve and Natasha.
It’s not just that Peggy’s had a full life since Steve disappeared from it, but that Natasha, Bucky and Sam are who they are because of him. If Steve were the main character in his own story, it would prioritize his character growth over any regressive nonsense with Peggy - and because he values her, the things she says would actually matter to him in return. In the last film where Steve was the main character, this was the case. Steve was heartbroken over Peggy’s age and dementia, but he respected Peggy enough to try and take her advice. (He had even tried dating, and “she’s not Peggy” had never been the reason he hadn’t connected with someone - but a “lack of shared life experience”, he says wryly to Natasha. That’s about Steve moving 70 years into the future - not Steve literally being stuck in the past.) In a good story, even when a man is the main character, his protagonist role has room to give his supporting characters stories of their own: that’s why the Iron Man franchise always had so much room for Pepper. In Iron Man 3 the climax of the film features Pepper saving Tony’s life - and then exclaiming in alarm, “Wow, that was really violent,” in accordance with her own values, not Tony’s: that’s the woman he loves, and this is why he loves her. So too it was when Peggy told Steve in Winter Soldier that she had lived a full life and regretted that he had not. Steve had gone to Peggy for advice as a friend, not because he had any thought that they could rekindle what they lost seventy years in the past. So why would Endgame derail Peggy’s life to make room for Steve - after steadily diminishing Steve’s satellite cast and writing Natasha off completely? It’s not about Steve getting the girl. It’s about Steve getting out of the present, so he can remain a symbol of goodness in the past. Unmourned, un-looked-up-to, and un-threatening to the legacy of Tony Stark.
