In October 1975, I bought a brand-new white 4001 for $270 and a beat-up EB-0 that I paid $100 for. Money was from working at Jack-in-the-Box. Played the hell out of that bass.
Wow, compared to the $400 a year later that's a good jump. On top of that my '77 which sold in '78 (I did not buy it originally but it came with the original hang tag) was $666.
Going for and sold for are two different things.
I've got a spotless 1972 Fender Jazz I can't seem to find someone with $5000 to buy it.
I think it's a combination of long time players getting older, young kids not picking up the mantle and learning an instrument and people just not having the money to buy the real thing when China is pumping out "same thing, just cheaper", instruments.
I seems more people would rather have 10 $300 basses than one $3000 bass.
I'd think the era of "bands" has definitely slowed down. I'm not a doomsayer that it's dead/dying, but it is no longer the absolute end all - be all juggernaut that it enjoyed being for decades. So that definitely would result in lesser interest overall combined with young folks having less and less expendable income. Why spend thousands on an instrument when your going to tweak it for hours and hours in a DAW with plugins that will make it sound entirely different. In that case you just need an instrument that plays easy and records smooth. Not knocking either side of it but if that was more accessible I'd have likely also not gone down the "GAS for thousands of dollars of gear for years and years" route.
I am 30 and a verrry late millennial. To me, 10 years ago later in college, I looked at 60's instruments as "just too far in the past to be feasible" because of the price and "juuuuust before the players I really idolize were playing".
To me at the time, I was desperate for an early 70's J bass like Geddy Lee but couldn't come up with the $1,500 to justify a ratty one when I was living on $15 a week for food budget. I imagine to the folks in my younger brothers age bracket (9 years younger), unless they're really into the 70's, would think 80's and 90's are the "close enough to their era, but while still not exorbitant in comparison and retro/cool".
Nowadays, I still like vintage instruments but am more weary. My '75 Ripper is rad, but the neck needs a refret and planing work. My '77 Ric 4001 just had it's pickup rewound. My grandma's early 70's Kay E-100 is essentially currently unserviceable from stripped out screws whens she owned it. It's a lot of money to be throwing down on what could potentially need quite a bit of TLC. That's a big ask for younger folks with less disposable income when they could rock on down to guitar center and play a used bass before taking it home. All that to say, I still want a mid to late 70's J with bullet truss, 3 bolt neck, and preferably not 3TS but finish doesn't matter. That and an Ovation Magnum I.