NavList:

Thanks for the fact check (and splitting my opening post).

It is unclear to me why middle (or, for that matter, geosychronous) orbits would be immune to the effects of collisions resulting in a debris cloud. I would agree that it is farfetched to think that a LEO collision would result in enough pieces with sufficient energy to pose a risk to higher orbits.  But a collision could occur in a middle or geosynchronous orbit, leading to these orbits becoming unusable.

In any case, there are other plausible threats to the GNSS constellations, as you point out.