Steve Rogers goes into the Quantum Realm to set right what Thanos has done. While in there he has to make the “ultimate sacrifice” to save everyone. What the Avengers don’t know is he finds a way back to the 1940s and has his happily ever after with Peggy while quietly flighting for justice when the need arises. It solves a lot of problems. Steve “dies” in a dramatic moment, but also gets a happy ending for him and Chris Evans that everyone desires. He could also come back if they need him.