After reading your extended view of mining operations I have a suggestion for dealing with depleted asteroids.

Asteroids form from a combination of lost cargo (jettisoned, cargo not retrieved in time, etc), debris from combat, the effects of ion storms. The exact ratios and rate of return would have to be balanced but this gives plausible reason for asteroid generation as well as taking care of "hey, where'd all the space debris go?"

The Ion storms give reason for the transformation of metals to different ores. Talk to a physics guy for a plausible explanation. I'm an IA geek.

Also, one side effect is that areas that become popular for combat due to lack of asteroids would eventually become choked. The same with popular botting areas. This allows the maps to mutate autonomously over time.

Play with it. Work your magic and see what you think.

fgettaboutit