That is not true. The tech required to maintain long-term living on Mars would be extremely valuable here on earth. It is directly relevant to many of our current goals for more sustainable lifestyles.
Much like the moon missions, the side-effect benefits that it encourages will likely end up making such a mission worthwhile for the tech alone.
I don't understand your criticism of starship though, would you clarify why you think it's not suitable?
Maintaining an adequate technological basis for continued survival on Mars requires, simply, many, many times more people than Mars could support. So, all the more tricky components and chemicals have to come from Earth. When those stop, the technological basis soon declines below the level that can support the population living there.
All the choices after that become very unpleasant, of questions about who gets to continue living. The supportable number declines inexorably to zero.
Much like the moon missions, the side-effect benefits that it encourages will likely end up making such a mission worthwhile for the tech alone.
I don't understand your criticism of starship though, would you clarify why you think it's not suitable?