I wouldn’t put it past a SoS Romney to run for President in 2020.
Romney would be enough of an opportunistic backstabber to run against Trump in the primaries.
I wasn't thrilled with Nikki Haley as UN ambassador, but that's a position of no consequence. Having Mitt as Secretary of State would be much worse - it's a position with considerable clout, and he (unlike Nikki) has the underhanded intelligence and cunning to do some damage.