I don't get the appeal of collar stays made from MOP, silver, gold, or unobtainium. What's the advantage over simple brass or even sturdy plastic? It's not like anyone ever sees them (certainly not the @#%$ cleaners who press shirts with the stays in place).
I've never seen plastic stays rigid enough to do the job very well. I don't see any reason why they couldn't be made, but I haven't seen any. The disadvantage to the various species of metal stays is that their sharp edges tend to damage the fabric that forms the stay pocket of the shirt. If the shirt lives long enough, the stay pocket may eventually blow out, and you're sunk. MOP stays are rigid and will not damage the stay pocket. On the down side, they're slippery enough that they don't stay put in the pocket as well as I would like (hey, Chuck, whatever happened to engraving a crosshatch pattern on the reverse?). As for nobody seeing them, you
will see them, and that's enough.