“ My point is Garmin wouldn’t have had to devote much shelf space or financial capital to have purchased enough spares to service all the units that will ever need”
It appears that Garmin did just that…. and ran out. Several years ago they stopped upgrading non-WAAS units to WAAS as their spares ran down. And then restricted repairs only to WAAS units as supplies dwindled further. And now it appears that stock is running dry.
I would guess one component that is hard to source is the screen itself. It’s not something you can 3D print, or build yourself. If the manufacturer doesn’t make it, you can’t get it.
Just like the displays for the KX-155/165. Nobody can make them anymore (kinda like a Saturn V…). Bendix/King does offer an alternative, replacing the original high-voltage display with LED. But that involves changing several circuit boards because of the voltage differences. This is something that needs testing and re-certification on a very low-volume product, resulting in a very high price.