Very good points, for sure.
But the market has changed significantly since 1986. The supply has shrunk and the prices are way higher.
As for the guns, I was told that a receiver part, which is the MG, can last forever is cared for properly.
As to the 100,000 number, that is only an estimate that I got from different forums. Estimates run from 100,000 low to a high of 250,000. Even at 250,000 that's still rare. I think only the ATF knows the actual numbers.
I'm surprised no one has made a FOIA request to find out how many transferrable MGs are in the ATF database.