But wait a minute...where then do the PGMs go after impact?

Do they fall into micro-black holes ripped in space-time by the force of impact? Do they meet their anti-PGM selves on a quantum level?

Not all asteroids are going to hit the Moon at enormous velocities because there isn't an atmosphere to slow it down. Stuff is wandering around this neighborhood on all kinds of trajectories and velocities.

In his book, Mr. Wingo notes the layout of the mining operations at Sudbury. They tend to be around the edges of the astrobleme. Maybe there's something to that, or perhaps go looking at the antipodes of the impacts. We don't know enough yet to say one way or another, but I certainly don't think the idea can be dismissed as absurd.

We know there are asteroids containing PGMs have hit the Moon because asteroids containing PGMs have hit the Earth. The stuff is there, the question is where to find it.

As for the asteroids being the best bet you're probably right long term, but I think the carbonaceous chondrites are going to be the early attraction, much moreso than metals. Capitalists are going to be freaking out about metals from space for a while. Biogenics are a safe compromise. Besides, we're still a ways from that happening. I'll bet we even get to Mars before then. ;-)