The technology and infrastructure needed to reach Planet 9, build a mining outpost, then transport everything back at a profitable cost is likely decades if not centuries away. That’s not counting the fact that Planet 9 is so far away that getting there with current technology is a lengthy affair in itself (the New Horizons probe wouldn’t even be halfway to a theoretical Planet 9, and it’s been flying for over 15 years!)
Not saying I don’t want this to happen - I would love nothing more than to have humanity push out into the solar system, but recycling or scavenging is infinitely more of an realistic option.