[It strikes me that the latest level RC from Ilford for instance has maybe only just turned 20 years old and maybe not even yet?

I have first generation Kodak and GAF RC prints, the Kodak prints are holding up. I think paper manufactures were (perhaps still are) concerned about claims that would not hold up in the field, leading to overall distrust of the new paper. Few users needed a commerical print to last for more just a couple of years anyway. Aside from testing in the lab no one really knew what the life span of RC was going to be so I think Kodak and other errord on the side caution. I dont see why a RC print will not last 75 to 100 years, at least in storage.