Sending and returning people to Mars is a problem that has far fewer question marks than having people stay there. A return mission is essentially an engineering problem: likely one of the most difficult ones humanity has had, but at least we know how to pose the questions which we must answer.
For a non-return mission, there are so many unknowns. We have barely a clue on how to sustain a human body on such a far away place nutritionally, psychologically and medically. If we were to send humans to Mars to grow old there, there will be so many unknowns that they will likely die quickly and helplessly due to some aspect of Mars missions we are ignorant of. It is running an experiment with human lives. Given that we know so little about the risks involved, I think it's unethical to send people there like that. Even if people choose for this themselves, it becomes at best unhelpful for humanity's view of space exploration to see people die helplessly on some remote world because we were running a grand experiment.
Sending people to Mars, and ending up having to say "Wow, we did not see that coming" is not something that will benefit humanity. We are not even close to being well-prepared enough for such a mission to become a success.
Moreover, return missions are a way to actually prepare the colonization experiment. Colonization can be the next step once longer and longer return missions give us the vital information we need about how living on Mars works for humans.
Return missions are something we know we can do. We know how to sustain humans for days in environments like outer space and the moon, places far more hostile than Mars (the trip to Mars will likely be far more hostile than actually being there on the surface). We just need to solve an engineering problem.
I also think you are overestimating our robotics. A trained geologist on the surface of Mars for a few weeks with a suitcase of instruments and e-mail access would give us exponentially more information than all previous Mars missions combined.