Steve goes back to Peggy and starts a new alternative timeline. One in which he’s married to her but there’s another frozen Steve. Frozen Steve, of course will still have everything stay the same; have to fight Thanos, go through the snap, etc in his timeline, and even go back in time to Peggy.
In that same alternative reality, Old man Steve shows up to avengers HQ with Peggy. They suit up together and go to the soul stone with black widow and Hawkeye. Steve is pretty much over Peggy at this point after being married some odd 50 years with everything she does driving him insane. She’s been on his arse about leaving his socks on the floor for 5 decades. Her meatloaf sucks and yet she still makes it every Wednesday. She got $12k on credit card bills. He’s basically over it, so he pushes Peggy off the cliff.
Black Widow and Old Cap go back to original timeline where we see cap joyfully staring at the lake pondering the first freedom he’s had in decades.
