Rubio's antiquated social views will be a major disadvantage by the time Trump is gone.
This. He's not going to look like a moderate savior when he's against gay marriage and the country is 65% in favor of it.
Some polls show 65% support already. By 2020, I can envision it at 67%, and if Trump is re-elected, it'll probably be at 70-72% in 2024.
I remember seeing that support for gay marriage has actually declined in the post-Obergefell era, as the SCOTUS action and subsequent SJW/liberal reactions unified Republicans against it.