Though Steve Rogers hung up his shield in Avengers: Endgame, a line from The Avengers reveals why the world needed a new Captain America. At the end of Endgame, Steve Rogers (Chris Evans) embarks on a mission to return the Infinity Stones to their proper timelines. By doing so, Captain America is clipping dangerous timeline branches before they start. However, after completing the task, Steve chose not to return from his time travels and lived his life in the past with Peggy Carter (Hayley Atwell). He shows up again in Endgame as an older man, passing on his shield to Sam Wilson (Anthony Mackie) as the new Captain America.
The Avengers was the first major team-up film in the Marvel Cinematic Universe. In the movie, Loki (Tom Hiddleston) attempts to take over Earth with the Chitauri aliens. To stop them, the Avengers need to put aside their differences and work together. Unfortunately, the effort isn't without loss. S.H.I.E.L.D.'s Agent Phil Coulson (Clark Gregg) is killed by Loki ahead of the Chitauri alien invasion. However, his death inspires the heroes to fight as a team. The group defeats Loki, solidifies themselves as Earth's Mightiest Heroes, and earns the right to celebrate with some Shawarma.
Though there's no replacing Steve Rogers, one line in Avengers shows why there needs to be a new Captain America. After being stuck in ice for 70 years, Captain America agrees to help in the fight against Loki in his first mission. On the way to the S.H.I.E.L.D. base, Coulson tells Rogers that modifications were made to his Captain America costume, which takes him by surprise. "The uniform? Aren't the stars and stripes a little old-fashioned?" he says. Coulson responds, saying, "With everything that's happening, and the things that are about to come to light, people might just need a little old-fashioned." The line strikes a chord with Captain America and shows why the MCU must always have a Cap.
Captain America is a constant in the MCU on which people can rely. In the Avengers, Loki's invasion gives the Earth a taste of the other-world threats that are on their way in the MCU. But, by having Captain America fighting for it, Earth knows it always has someone in its corner who's ready to do whatever it takes. In Endgame, the world is dealing with the reality of Thanos' (Josh Brolin) deadly finger snap and the hardships that also come after it's reversed. With Steve Rogers gone, Sam Wilson's Captain America is sorely needed to inspire people and hold them to a higher standard in the face of a world that's rapidly changing. He immediately proves this in his speech in The Falcon and the Winter Soldier finale about the responsibility of power.
Fans need Captain America too. Avengers: Endgame helped bring Phase 3 of the MCU to a close, giving fitting goodbyes to many of the heroes that made the first decade of the Marvel movies so successful. With so much change happening in the MCU, fans also need a constant. While the new phases of the MCU will usher in the Multiverse and different villains and heroes, Sam Wilson's Captain America will always be around to help keep things grounded and serve as the connective tissue between the MCU's past and present. The Avengers line still holds. Captain America provides the "little old-fashioned" people need.