I think Rob S is more concerned with stocking up on parts that may be harder to get once the manufacturer no longer needs to make them. He's not necessarily looking to replace the items in question right now. For things like a cat, I think there isn't a problem because there should be a billion aftermarket units that can be welded-in. Other parts on the list may be more specific to his model/year.

I know for my SVX I have a stock of fuel filters (because only the OEM fits perfectly), radiator caps, key blanks, door sill panels (made with clips that like to break), grilles, and I plan on adding a MAF sensor to the list since that's already fetching over $300 from dealers.

It's similar to the cache of empeg VFDs, remotes, harnesses, tuners, and metalwork I've amassed.