Really? I think he was strategic (and smart) to sit out this time, but I don't see why he wouldn't take the plunge in 2020 if Hillary wins. Why not? He'd only be 61, that's not that old in political terms.
If he wanted to be President, he wouldn't have signed RFRA, especially in a cultural climate where Christianity is declining.
I think he thought the RFRA would help him significantly in the GOP primary (he certainly implied that he wanted to run), but it turned out to be so controversial in general that he would have gained too much negative media attention to make a good run.