Avengers: Endgame was one of the biggest cinematic events of the last decade, so much so that fans continue to discuss it nearly two years later. Endgame seemingly brought an end to the stories of multiple members of the Avengers that the audience has come to know and love, whether it was through death or other means.
Though some of these endings were accepted by fans and agreed upon to be good conclusions to character arcs, others were not. More specifically, the ending of Captain America's arc was hotly contested, because it feels unsatisfying and contradictory to the themes of the rest of his story. It's strange that it seems the Russo Brothers (directors of Endgame), who were the very people that set up the direction that Steve Rogers's character would go from Captain America: The Winter Soldier onward, were unable to properly finish the story that they started.
Captain America's conclusion in the Infinity Saga is simply him using the Infinity Stones to go back in time (perhaps creating a different timeline) in order to be with his love interest from Captain America: The First Avenger, Peggy Carter, and live out the rest of his days with her. The scene where we see him and Peggy together is quite literally the last one in the movie, and so it's not only the closing chapter for Steve, but for the Infinity Saga as a whole.
The problem with this ending is that it doesn't make sense, and doesn't fit with the character journey that Steve has undergone up until this point. Steve's arc through all of the Captain America movies, as well as the Avengers films, is about moving on from the past. He is initially a man out of time, and has to deal with that adjustment in the early films, and as they go on, he seems to become more and more comfortable with his place in the modern day. He forms connections to people and learns to accept that this is where he has to be, and he seems to be okay with that.
Peggy's death in Captain America: Civil War seems like a sort of closure, as one of his last remaining connections to that time period he was originally from is gone. He has to let go of the past in order to protect the world from its present dangers, and before Endgame, we are really given no indications that he feels a longing for the past again.
Steve wasn't the only one who had a whole arc about moving on, because Peggy Carter had an entire show that detailed her struggle of moving on from Steve and the events of The First Avenger. There is a symbolic moment in Agent Carter where Peggy dumps the last remaining vial of Steve's blood (initially kept for scientific reasons because of the super soldier serum in it) into a river, letting go of him both physically and emotionally. She moves on and starts living her own life and becoming her own person instead of just a side character or love interest in Steve's story. She even falls in love again in the course of the show, with fellow agent Daniel Sousa.
The fact that the writers felt that the most fitting and satisfying end to Steve's story would be to have him go back to be with Peggy undermines both of their stories. They have both made progress towards moving on and living their lives, only to have all of that be seemingly ignored and reversed in favor of some fairytale, white picket fence ending. It feels like a cop-out, to have a character be able to literally go back in time to be happy instead of just finding happiness in their current moment, because dealing with living in the modern day and finding his place in the world is what Steve's journey has always been about. It would be satisfying to see him finally realize he is content in the present, rather than undo all of that character development and bend time and space to literally go right back to where he started.
It's not just the characters of Peggy and Steve that are complicated by this choice. Having Steve go back to be with Peggy makes his already awkward kiss with Peggy's niece, Sharon Carter, in Captain America: Civil War even weirder, since his going back in time technically makes her Steve's niece as well. This romantic moment that felt forced to begin with is just made even odder by the fact that Steve apparently had not really moved on from Peggy when he kissed Sharon, if he was willing to go back in time to be with her.
It also doesn't make sense that Steve would leave Bucky Barnes behind after spending three movies doing everything in his power to get him back and protect him from HYDRA. Bucky is integral to Steve's character, and to the Captain America trilogy as a whole, and it's a strange character choice for the writers to make that just as things with Bucky calm down, and he has finally broken free of HYDRA (for the time being, at least), Steve just leaves. If Bucky has to deal with the consequences of being out of his time and living in the present, why doesn't Steve? It simply doesn't make sense for Steve to abandon his life-long friend for a woman that he kissed once 80 years ago.
Of course, this end was probably chosen because Chris Evans's contract with Marvel was finished, and he had expressed a desire to move on, which means that Cap wouldn't be a part of the MCU anymore. The easiest way to do this (other than killing the character off) is to send him back in time, but it might have felt more serviceable to the story if they had found a way to have him stay in the present, but just retire from being a superhero. No matter the reasoning for this choice, it simply feels like an unsatisfying end to the arc of a beloved character, and makes any poignancy in Steve's story fall a little flat.