I agree 100%- FA weapons won't go down. Not only is there a limited number, but the number actually DECREASES.
They get damaged, get tied up in inheritence battles, etc.
They even get lost to the state sometimes due to not having them in the will correctly, and/or the paperwork not being ready in time to facilitate transfer to family members. There are probably more than a few out there that never got transferred following the owner's death. The new owners are probably ignorant of the laws. Once found, and confiscated, there's another one off the books. I've had people show me guns that were full auto, and they never knew it until I told them. It's been quite a while, however.
Every few years, the topic of another amnesty period comes up mostly just for this sort of thing. I personally think it's a good idea, as people who have such things illegally and are unaware of it, would find out. The BATF&E would benefit by learning the whereabouts of an unknown amount of stuff.
It's probably rare, but I've heard of inheritence cases where a benificiary chooses not to go through the so-called hassle of transfer and ownership, and surrenders the gun.
One machinegun-related issue that I think might (well, I hope) change is magazine prices. I have two of the relatively cheaper SMGs- an S&W 76 and H&R 50 Reising. But, the magazines they use are some of the priciest. Although there has been an influx of much cheaper Swedish K/Port Said/Carl Gustv M45 magazines in recent years, which are easily converted to S&W 76 use.
Hopefully, Ken Christie can, and will, make up a bunch more Reising magazines if the ban goes away.