First, what's the return on that?
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.
There is another way. Irrationality. People spend a lot on religion. Like a whole lot.
What if there was a faith system of ultimatley going to interstellar medium. You have faith, you automatically pay, like the rest of the people and you dont question it. You get tax breaks. It will help you in the end of times or something.
Just decide the ultimate goal to be interstellar medium touching in all directions.
You are a farmer? Well now you continue to farm to feed budding spacers. You are a game dev? Well, people are going to get bored in space, continue developing games for the ultimate goal.
My response to the money aspect of this it's just like any other business: money needs to be invested, and then a return will be realized. Resource extraction (i.e, asteroid mining) is one obvious example.
The human compatibility issues with microgravity are well known, as is the solution, which has even been proposed by NASA: centripetal force to create 1G for the astronauts.
As far the the radiation goes, we do indeed know exactly what kinds of radiation they would encounter. And the easiest way to shield humans from it in space is lots of water, or metal. We know this from extensive real work done on earth re: nuclear power plants.
The real issue is money, not technical feasibility. Once the dough rolls in from asteroid mining, it bootstraps the financing issue and pays for itself many times over.
Asteroid mining is one thing. Exploring the nearest star system is science expedition where the payback is in societal scientific knowledge and subsidizing technology development that is then made available here for various things (eg a lot of the space exploration tech in the 60s made its way into consumer tech)
https://www.nasa.gov/humans-in-space/the-human-body-in-space...
NASA seems less sure than you do. And considering we have to get to the asteroids before we even start to think about mining them, talking about the money from asteroid mining is putting the cart before the horse.
Class 1 civilization has a lot of resources