I think it comes down to appreciations of the simple classic bike design, whether it is an old or new. For me anyway, something about a classic triangle frame, in lugged steel with quill stem and either a drop bar stylem"english racer", or even 80's MTB style is simple design elegance and proportions.

modern bikes, road or mountain are highly functional, but in general ugly

I think as long as people appreciate the style, there will be appreciation and interest and production of classic bikes. and some of this will also be interest in vintage.

and perhaps a bit idiosyncratically, I am totally ok with brifters/sti, dual pivot brakes, etc as they fit with the design.