I definitely agree with you, however, I think needing to become self sustaining on earth is a goal that would be well served by trying to design a self sufficient system for mars.

Earth is big enough that it's really easy to forget we're all in the same fish bowl. Entire cities can flush their shit down the river and as far as they are concerned, nothing bad ever happens to them. The scale of earth makes us blind to the problems our actions and methods cause. The ecosystems also do quite a bit to protect us from our own actions

You can't ignore externalities in a space colony. Everything must be accounted for. That is what makes it so difficult to design for. Any small amount of waste will still accumulate over time and eventually becomes a problem.

The tighter scope and strict requirements of a space colony would make it easier to actually objectively measure how sustainable it is. You would know exactly how much external inputs you are delivering each year. We can then take the lessons and technologies that are absolutely required in a space settlement and use them to inform how to better be sustainable on earth. For example, solar cells used to only really be used on satellites, not because they were great on satellites, but because they were pretty much the only option that could stay operational for years. Now PV power generation is helping countries all over the world become a little more sustainable. The harsh requirements of space make us better at problem solving.

I totally agree that earth is our only option for species survival though. Anyone selling Mars as a "backup" for humanity is either delusional or a con man. I think developing the capability to keep a settlement on Mars is a worthwhile endeavor, but there is no way for humanity to thrive there. Any large scale catastrophe on earth will still be more survivable in select pockets on earth than anywhere on Mars.