The used market is a diverse thing. Vintage silver-face used gear is having its moment, selling for multiples of its MSRP in the 70's. This is a select slice of the market, attractive equipment with vintage slide rule tuners, balance, bass and treble adjustments, and enough auxiliary inputs to accept a modern DAC and streamer and a tuner preamp and one or two headphone jacks. Of course these were upper end of a mass-market brand production, not modern ultra high-end used gear which is priced very differently. Who can say how much longer any of that will last. Gear made in the 1980s and later is vulnerable to repair/replacement parts unavailability as ICs become obsolete and salvage parts become scarce. I suspect that except for brands like McIntosh and similar well-supported legacy brands, much of that period gear will be recycled and not restored. I once bought a legacy Luxman receiver, the ones with the Lucite panel buttons and the motorized receding faceplate. Even once-expensive gear like that couldn't find a market to service it.
Then there is the possibility of loss of whole technologies to factors yet unknown. Vacuum tubes are one of those technologies vulnerable to shifts in production even now. Imagine a future where power consumption of every household device might be sharply controlled and products like inefficient and heat-producing amplifiers might be heavily taxed or even outlawed. A desperate world where warming becomes an existential reality that cannot be ignored may impose such bans.