> If I were a billionaire space tech guy, I'd be working toward O'neil cylinders instead of Mars colonization.

While I agree with you in the longer term, I don’t think we’re even close to ready to build O'Neill cylinders. First thing I’d go for would be a lunar colony. Lots of useful resources, relatively short trip home in emergencies, a way to get experience working in real vacuum, and non-rocket launch systems could already be built there with current tech.