I have had a very similar discussion / debate with many others in the ’vintage’ audiophile community... you know, the ones that have storage lockers filled with 50 - 60 year old equipment and won’t part with any of it because it is an "investment"...
Mark my words. In the next 10 - 20 years the vintage branch of the HiFi market will dry up faster than that firewood you have stacked up next to your garage. And the vast majority of these ’investments’ won’t be worth the cost of having 1-800-GOT JUNK come and take it all away (which is what your kids are going to do after you are gone...) And I would think similar to the modern audiophile industry as well.
Look at the ’musclecar’ market on a large scale to see what the future holds. Yeah, if you have a fully restored Hemi Superbird or a 70 Chevelle SS LS6 you are still golden... but that 1969 Nova with the 350 that cost you 20K to do a full-on rotisserie resto in 2015..? You aren’t even going to recoup that 5 -year old restoration bill anymore... in fact, you would be luck to get back HALF that cost today.
Now, in fairness, there will ALWAYS be at least some demand and interest in the most high end and rarest audio equipment, whether of the vintage or modern variety (or classic musclecars, for that matter). And there will always be high net worth individuals willing to spend a premium in order to acquire said equipment. But all those folks with the Marantz receivers and the 1980 Klipsch Heresy speakers and the JBL 100s, etc. who think they are sitting on a goldmine of ever increasing value...
IMO the time to sell and get what you can is fast approaching. Holding for longer than maybe another 5 years will almost certainly bring no better than current prices - and quite possibly you will have missed the peak, and maybe even less than today’s pricing.
There just won’t be as many people interested in this stuff anymore. Period.