When a part is that valuable they don't wear out, they have new metal added and they get ground or machined down to spec. Metal wears thin or breaks, blob of new metal fills in where needed and you machine it down.
You can essentially build all new parts, but not from scratch, you start with what is left of the old metal so it is technically still the same part just "fixed".
You can fix something indefinitely, it is just a matter of adding more metal.
Since the regulated parts are not the parts that contain the pressure, such as the barrels or chambers, the regulated part can constantly be pieced back together rather easily.
The only real reason the registry gets smaller is people who have the firearms confiscated or lose them somehow.
Every now and then some guy with a collection of machineguns gets busted for some charge, and all those guns can end up confiscated from the prohibited person and leave civilian circulation.
Considering some people have entire vaults of registered machineguns, a rather large chunk can be removed from the registry when the right person goes down.
You hear about it sometimes and it makes the local or regional news, and I am sure there is times it goes unreported.
So the registry does shrink, but it shouldn't be from wear unless someone is naive enough to throw it away when it wear out.
