You are talking about massive investments to shoot off into space never to return. Who's paying for that? The only way you do that is if you're so fucked, it's your only option and the profit in it is the leaving.
Not to mention, we need to solve the problems of living in space. Which we haven't yet. According to NASA. The space people.
And it very well could be an insurmountable problem. We do not know. We do know that living in microgravity fucks you up. We know that radiation fucks you up. But we don't even know all the types of radiation one might encounter.
> But if you get to the point where you are turning the asteroid belt into O'Neill cylinders
That right there is an example of "solve this impossibly hard problem and the rest is easy". We are nowhere near doing anything close to that.