It’s hard to understand how a small piece of plastic could acquire such value .
Pure scarcity value. Rickenbacker themselves have a like-for-like policy on broken TRCs. They are also the one bit of the instrument that says 'Rickenbacker', so can be used to verify your instrument or dress up a copy.
I have a Chinese 3D TRC on my 4003 bass. The original pickguard was damaged by the previous owner, so I bought a Chinese repro unit.