If we start mining the Moon, or asteroids, then...the good news is that would could just ruthlessly strip mine without worrying about the ecological damage because these are all dead worlds that dont have native flora and fauna, and therefore have no ecology.
The bad news... after you dig up some high grade iron ore on 16 Psyche, or the moon, then how do you get it to earth?
It might be easy to get bulk cargos of pig iron off an asteroid because asteroids have virtually no gravity, but then how do you land big bulk loads of ore back onto the earth?