I didn't say that they were. The fact that a single well chosen asteroid can represent ~5 times the annual global output of a given metal is a nice side effect

I didn't say that they were. The fact that a single well chosen asteroid can represent ~5 times the annual global output of a given metal is a nice side effect which happens to make the whole thing financially viable. The goal is to bootstrap the industrialization of space as a prelude to the establishment of permanent colonies throughout the solar system.