... They could use the quantum tunnel to de-age old Cap back to his age when he left like they accidentally did to Scott Lang...
I'm also now pretty sure that Cap did indeed live in the regular timeline in secret, was Peggy Carter's secret husband all along, and that's why she never had children (as that would massively disrupt the timeline). There's also a fan theory that one of Peggy Carter's pall bearers in Civil War is old Steve. You only see one of them from the back, he has grey hair that's styled like Steve's on the bench in Endgame.
