Well, when playing the smartass as I did, you better be correct

As for the "risk" of base quality rising, I don't know. Those who are able to afford 10 mineholes and high enough STR mining char are most likely big cities who knows what they're doing. So, unless you're one of those, you are already far behind in the run for quality anyway.
Plus mining straight down for 10 layers involve a lot of luck to find ore easily. Most likely you'll have to dig around a lot to start finding ore. Only a matter of time though, but the effort needed make it seems like a reasonable and "balanced" feature.