Hmm, there are really two parts to the OP's post:
- C-Prints deteriorate rapidly
- Therefore making C-prints is a waste of time
On part 1, it looks like experience is sharply divided, and that there are enough variables that it's hard to make generalizations. But based on the OP's and other observations, yes, C-prints can deteriorate faster than one would like. But it's not necessarily true, and with good processing and storage, they can last a lot longer than the 30-odd years that collection lasted.
But even so, it's an arguable point -- you're not guaranteed a 100-year point.
My real problem is with the suggestion that if it's not archival, it's not worth doing. The week before Christmas, I went to a performance of Handel's
Messiah -- it's an oratorio I love for its musical and lyrical richness. But you know, within milliseconds of the end of the final 'Amen', the echoes had died away and were completely irretrievable! Clearly nobody should waste their time singing or playing instruments!
Alright, you can reasonably say that people have different expectations from a collected photograph than from a musical performance, but archivability is not the be-all and end-all of whether a piece of art is worth making, sharing and enjoying. Everything comes to an end. Everything. Maybe c-prints fade in a few decades (and maybe not, depending), and silver prints in a century, books in a couple of centuries (if well printed and well bound), and marble sculptures a couple of millennia. But they too will be dust. So how long is 'long enough'?
I make c-prints because I enjoy making them, seeing them, hanging them, sharing them. 'Posterity' will have to make its own arrangements.