Anyone who is paying attention knows that Starship is mostly going to be a launch vehicle for Starlink. It's very unlikely that the upper stage will ever support external payloads.
Why wouldn't they make it for external payload if they get the cost per kg lower than F9?
Running starship only is going to be cheaper than running both rockets, except if the economics of starship are worse (in which case, it would not be used for starlink either).