There is no doubt that that newer bicycles offer superior performance and a more pleasurable experience for most cyclists but many can get by fine on vintage equipment. It all comes down to how much you value features like nine 18 speeds, brifters, profiled cogs, disc brakes, low spoke counts, etc. You've already noted the weight decrease. Often, most of these features can be had at a fraction of the price by buying something only a few years old. There's also much to said for buying something totally lacking in these features, especially if it is something you lusted after in your youth but couldn't afford. Often this sentimental bond can outweigh the technological advantages. Everybody is going to having a slightly different point where the price increase does not offset the technological advance. Of course, the common solution to the dilemna is N+1, with many members owning multiple bicycles spanning a wide range of levels and technological eras.