I would take an NRS boat any day of the week over a PVC boat (I have both), but there may be more to it than technological development. It might just make better economic sense to make it the way they do now?There seems to be a tendency to imbue old boat designs, fabrics and coatings with almost mythical properties; but for the latter there has been decades of technological development. So I'd be inclined to give the chemical engineers and raft makers some credit here, because behind the comparable performance aspects we end-users perhaps laud most there are also major considerations/trade-offs in the material sciences regarding durability, degradability, and/or say resistance to UV damage.
I own an Avon from '87 and a hyside from the early 2000s and the hyside has some design improvements but my unqualified opinion is that the Avon material just feels better and more supple despite being 20 years older.
I think the old fabric is better but not because I'm nostalgic about the old fabric... I've only owned these boats recently.
Just my 0.02