It's like saying we should colonize Antarctica because then we'd learn things that would make it easier to colonize the North Pole. Doesn't really answer the question.
So far, it's private money moving that direction. It's not tax dollars. If some uber rich asshole wants to spend his money on this, so be it. It's his money. If it doesn't work out, okay, we learn from it. But not going is just not going to happen. It's part of the human experience to ask "what's next" and then do it. Less adventurous can sit at home. The meek shall inherit the earth, the bold are going to space!
The only way I can see large-scale Mars colonization happening is if there's a strong economic incentive for that. It isn't going to happen just because a few people want it to, or think it's the "next step for humanity".
[1]: https://twitter.com/elonmusk/status/1217986505513172992
[2]: https://twitter.com/elonmusk/status/1217993568482025472
We've had the technology to colonize Antarctica pretty cheaply for quite some time now. I don't see any self sustaining cities there; just small research bases. Why should Mars be different?
Musk talks frequently about a self-sustaining Martian city becoming a "backup for humanity" in the case of some global extinction-level event (Asteroid impact, nuclear war, etc), which is all well and good. But who's going to pay for it? In order for Mars colonization to actually happen, it needs to be not only affordable, but profitable!