I don't think it's oxidation of the clearcoat that's the problem. It's oxidation of the metallic particles in the paint itself once the clearcoat degrades. Nonmetallic colors don't need as much protection because their components don't oxidize as readily.

That's correct. I had a dark bronzey coloured car many years ago on which the upper surfaces ended up a chalky colour. This was in Papua New Guinea, so a rather harsh environment.

I think polishing isn't a good thing for metallic paints as polish removes a tiny amount of the clear coat each time. Waxing at least adds a protective layer on top of it. On my last few vehicles which I've bought new I've gone with a protection package which includes a clear topcoat. No waxing or polishing ever needed; just wash, rinse then chamois it dry.

I think the interest is waning in metallic car colors...

Oh yes, I'll pay that one.