If it happens, Romney won’t win, and I don’t think it will happen. These days, when a candidate is nominated by a major party and loses the general election, he is NEVER given a second chance. The last candidate who got a second chance was Richard Nixon, in 1968.
True, Nixon lost in 1960, but in 1968, he was still his party’s most recent two term VP, which is BY FAR the most common pathway - almost always guaranteeing nomination.
History shows that Pence will almost certainly get the nomination in 2024, if he chooses to run.
Romney has no such qualification.