The vintage bike ‘investment’ bubble is going to burst harder than Bitcoin very soon.
Yeah, I agree to a point but I'm not sure how much of it is about "investment" though to be honest. Mostly I think it's just nostalgia and people wanting to buy bikes that they couldn't afford back in the day.
In road bikes in particular it's quite different because you can buy a 20 year-old Klein like above (or CAAD, etc) and still ride it without it just being for novelty. Hell, with threaded BBs and rim brakes, you can put a full 2021-spec build on a 20+yo frame and have a sick bike with character that isn't dramatically out of date like a MTB of comparable age would be.