There's always the chance they erase Jon in the present and go back to a time before he was born, but the character remains in Legion. They could say the Legion Jon comes from further in the timeline than the DC present. Jon almost never interacts with his parents and when he does he's cryptic about his true identity. Didio would call it a compromise between the Jon lovers and Jon haters, while anyone burned by the age up will lose out completely. I'd hate it, but I can't pretend that isn't a possibility.