You're using linear projections. Elon Musk's entire goal with SpaceX is the Interplanetary Transit System. I also think that it will be a boon to multinational corporations who want to perform research/experiments that are either not possible, heavily regulated, or outright banned on earth.
> 1. Using the mined materials for space construction. But why are you doing space construction in the first place? So you can do more asteroid mining? So you can do more space construction?
Yes, exactly. Space exploration is a bit limited without construction in space. There's also the likely scenario that as the world becomes wealther, our consumption of technology will also grow. Look at the bet on AI and the huge capital investment necessary to run ML/DL projects. A combination of increased demand and an increase in global wealth which raises people's standards of living is also going to increase the cos of mining as people's environmental awareness is raised.
SpaceX has a plan for the colonization of Mars, so yes, to do more space construction.
https://www.rt.com/news/310170-platinum-asteroid-2011-uw-158...
With they retain their mining rights and sell carefully, or will the price of platinum crash?
With a government steal it and will there be wars over it?
Or maybe you'll get a resurgence of nation-state-level companies like the old Dutch East Indies company, as well as piracy.
Which basically means a sustainable colony must produce a ton of energy. Good news, producing energy in the space is actually cheaper than on Earth: you have a lot of solar, and nuclear is no longer a hazard.
The economics of asteroid mining only make sense if you're:
1. Using the mined materials for space construction. But why are you doing space construction in the first place? So you can do more asteroid mining? So you can do more space construction?
2. You've found an asteroid made of pure unobtanium.