Both Ryan and Romney are both totally “free to run” anytime they want - 2020, 2024, etc...
They were both free to run in 2016 too.
You may disagree with Ryan right now, but you have to realize that he still is fairly conservative. He was mentored by Jack Kemp, who probably should have won the ‘88 Republican nomination over Bush. Kemp may have been a better president than GWB in ‘00. He would have probably beaten Gore by a landslide. Ryan is probably just worried about his future, than the American people’s future.