Thing is though, we're not going to work out how to build a sustainable colony on Mars on paper, then send people (or robots) there to build it sight-unseen. We'd build prototypes here on Earth first, like how we tested out the Mars rovers in Earth's deserts first. The propulsion technology to get stuff to Mars is largely a solved problem by now. What we need to solve is what to build when we get there, and how to get it built, which means developing robots to build sealed "biodomes" or whatnot in inhospitable places here on Earth first -- to test the technology to make sure it will work when we strap it to a rocket and send it to Mars -- and then, by the time we're ready to strap it to a rocket, there's not a whole lot of need to actually do that anymore, besides just "because it's there".
Which is still a perfectly fine reason in the long term, and if romanticism about people on Mars gets the technology funded and developed that's great, but one way or another we're going to be building Earth-bound versions of it first, and those "prototypes" themselves will then have already solved any practical problems that might have motivated the Mars mission.