It's true I'm thinking of the standard "boots on the ground" approach rather than a hybrid with some automation (humans only in orbit), etc. But this is what most everyone seems to mean when they say "colonize Mars". And of course, without boots on the ground, it's certainly less "fun" and interesting from the public's perspective and would probably get less support.
So anyway, speaking only on the "boots on ground" approach (wrong or not), I'm still skeptical that the problems are really solvable on a long term basis. I'm aware of the redundancy strategy space programs take - but I still feel like it's a rather thin layer of protection for a multi-year or permanent mission.
Manufacturing in space, I don't see any chance of anything significant at all in our lifetimes. A good primer on how hard manufacturing from scratch is (on Earth!) - "I, Pencil": www.youtube.com/watch?v=IYO3tOqDISE
i.e. Okay, so we've grabbed that huge metal/ice asteroid - what are the next 100-500 steps and how do we do them in the environment of space? If it is possible to do anything useful with that rock, surely it'd involve a lot of "cheating" and bringing other stuff from Earth.
If just a simple pencil is that challenging for humans to produce on Earth, requiring a large free market economy, I can't imagine a small number of people building anything of the required complexity and quality required off-planet. So to me, other than a handful of feasible production ideas such as fuel manufacturing or 3D printed items, it doesn't seem to help much. Even something seemingly simple like a solar panel (which I doubt is simple at all), or mirrors, I am guessing would be extremely difficult to produce without a ton of infrastructure in place already.
It was mentioned that some of the environmental concerns I raised (air going bad, crops dying off, etc.) are all solvable, and that I don't deny. Each problem seems to have a feasible solution. But when taken as a whole, isn't the number of problems overwhelming? Even just monitoring for problems seems challenging. The plants aren't going to tell you if something bad leaked into their water supply, dangerous odorless gasses can only be detected by sensors if they're onboard and working... (it's not so hard to imagine problems). Miss just one critical problem...
So, while I like what SpaceX is trying to do and all, are Musk and other "leaders" looking to do permanent settlement off-world uninformed and overly optimistic? Am I overly pessimistic? I feel like 20 years from now we'll end up realizing that they were all at the "peak of mount stupid", and at best we'll have a small ISS-like moon base, underfunded with an uncertain future.