Overall, I believe that there is 0 reason to even attempt it for now. The Earth is in the middle of an existential crisis for human civilization as we know it, and all of our resources are required at home. We need to rebuild the global economy from the ground up in a way that will reduce global warming. Mars is no escape from this, certainly not in the next centuries.
Any cent that goes to a Mars colony would be better spent on Earth. If not for combating global warming, then at least for building self-sustaining colonies on Earth that could become a refuge for humanity if all else fails, or if we get struck by some asteroid. And this would not only buy us guaranteed survival, it would also act as a perfect model for a Mars colony is some far future - if we had actually built one on Earth, with all its requirements, building one on Mars would certainly become much easier.
Until and unless we know for sure that we could build truly permanent high-tech human settlements on Earth, there is no point in trying to in space. There is nothing to be gained from Mars if we can't build a permanent colony there not dependent on Earth resources, and we will not be able to build a permanent colony there unless we first build one on Earth.
I disagree that there is “nothing to gain” and in fact I think there’s a lot humanity could gain from the development of regular space travel and settlement of other planets.