Slow steady evolution, single parts changing over several years but backwards compatibility being retained. You can still put quite recent groupsets on most of those old frames, save some rear end respacing. Whereas in recent years there has been a proliferation of standards, overlapping and incompatible that even after a few years specific parts can become harder to attain than for a bike fifty years old. It wasn't a reference to performance evolution, just the proliferation of headset and BB standards etc. An oldschool friction shifter doesn't pose the same incompatibility issues as indexed shifters and derailleurs. So you might commit the sin of pairing two style incompatible pieces together but they will still function until such time as you decide to correct the style error. Buy the wrong part today and you buy twice etc.