ISP is like fuel milage, and the economics change drastically when there's infrastructure in place. Having an abundant source of propellant near a celestial body with a much smaller gravity well (Mars) could easily change the economics and practicality of which fuels make sense for different missions.

Put another way, we might one day look at Starship as a big rig semi truck (though the ISP of Raptor is actually really good while a big rig's is very much not). The fuel efficiency isn't really that important.

I think (or hope at least) we're far away from being able to really see how much space access is going to change if Starship actually pulls off what they're trying.