Colonizing the solar system for resources is something that's said by people who don't understand astronomy or space technology (like Elon Musk).
The solar system readily provides both the motivation, and the means, for technological colonization. It quickly pays and provides for its own utilization.
I do agree Musk's projections for biological/human colonization appear to be unrealistic. Just one challenge: it isn't clear humans can reproduce in non-Earth gravity. That is an extreme impediment.
Celestial mechanics suggests it can hardly be economical to move large amounts of matter around a solar system for economic goals considered in the current capitalist societies. Nor the risk, nor the energy needed are in line with the benefits.
A fully fueled ship in Earth orbit can go to Mars, land, and return. Same with most other round trips.
Not to mention, fuel can be generated in the belt outside any noticeable gravity well.