+3 Half my vintage bike buyers are younger than the bike. They just like the style, construction, aesthetics. I think there will always be a good market for lugged steel framed bikes.

While most vintage MTBs have not gone up in value (there are EXCEPTIONS!), parts off these vintage MTBs are doing quite well. My next bike project is to dismantle one I built last year. The thumb shifters and crankset alone bring more than the complete bike! Have you seen the prices on Suntour bear trap pedals lately? I've already moved the wheels and tires to another project. And some vintage rigid steel mtbs, think bi-planer forks, solid chrome frames, roller cams, etc., are starting to do very well.

That Specialized Hard Rock is not a rigid frame, with a mediocre suspension front fork, those are still dropping in value around here.

Heck, the High Sierra wasn't even the top Schwinn model that year!

Vintage Schwinn High Sierra 19" Mountain Bike 1987 Black Chrome 4130 RARE | eBay