The benefits of a Mars colony to those of us smart enough to stay on Earth are likely to be immense. To achieve such a thing is absurdly difficult, so it requires a wide variety of innovations. It requires finding ways to use limited resources in far more efficient ways than we've ever done before. It requires recycling of everything you build into other things you need. It requires massive improvements to 3D printing to create things on site you can't wait for delivery on. One Starship a day is minuscule compared to for example the total weight of the 230,000 cars per we produce per day, so what we learn and innovate from forced conservation building the colony can easily can make it a net benefit to Earth's resources.
Yes, you could develop those technologies on Earth for Earth's sake, but nobody will because there's nothing forcing them -- just like all the tech that resulted from Apollo that wouldn't have happened without the visits to a barren space rock.
When it comes to the humanity's eggs in one basket argument, it would be dramatically cheaper and more effective to build self-sufficient colonies deep underground that can survive even a comet strike. But that'll never happen, because nobody dreams about living in such a place. Leaps forward happen when they're forced to happen, be it by a war or a space race, and personally I prefer the latter.