I like my vintage silverface stuff, including Marantz stuff, ... it's fun to look at and nice to listen to. However, there are alternatives in good new or newer analog equipment that will blow away my vintage stuff, in every way except value. The '70s stuff had good specs when new, and if properly restored still does. However, you really can't (IMO) compare a 2370 to what you'd buy in (for example) a new ADCOM for the same money. It's analog, it's going to be built as dual-monoblock, it is made in China, ... which is in some ways the same as made in Japan in the 1970s.
In the late '70s Marantz was considered pretty good, better perhaps than Pioneer, one could argue whether Yamaha was better sounding, but in any case it was not even in the same room (in my local stereo shop) as the McIntosh or Conrad Johnson stuff. Really upper mid-level, ... pretty much like a Corvette, not a Ferrari.
I like the comparison to muscle cars, because I agree that it is very similar. When my generation got to the point where we had extra money, we started to buy things that we couldn't have when we were younger, muscle cars! The Ferraris were always worth money (Conrad Johnson, McIntosh), but Mach 1s and Z-28s from the 60s were being crushed in junkyards. Hell, I met a guy at an auction that rescued an LP-400S Mura from a junkyard!
When my parents were in their 50s and 60s they were buying the cars that were hot when they were young, those prices have plateaued as they stopped buying cars and dying. When my generation is toting oxygen bottles the '60s and '70s cars will plateau, as will '60s and '70s stereos.
Sure there is an export market, and there will be future generations that will continue to collect, but there will then be a new generation of products that will become collectible, ...
As far as supply, I have a feeling that hoarders are not all collectors, and as these pieces appreciate there will be more of you who look at that pile of equipment in the garage and see dollar signs, sell much of it for a tidy profit and keep a few nice pieces for yourselves. I was in a stereo shop last winter where the guy showed me 3 Pioneer SX-1980s in museum condition, has two Sansui 9090s on the service bench for testing speakers, ... figures it's part of his retirement investment and has no interest in selling. Those 1980s hit 5-figures he might change his mind, might not.