I have a Fender Rhodes Mark 1 88-key electric piano. No lid, no pedal, no legs (I keep it on 2 Ikea stools). In decent condition otherwise. But it plays great and sounds great. It weighs a lot and it takes up plenty of space. I almost left it in L.A.
I have a Fender Mustang bass from the 60s. My Rhodes is likely from the 70s. I can get either fixed to "like new" condition for a few hundred dollars and fully restored for not much more than that.
I used to have an Akai S1000 sampler. When I needed to replace the display (a critical part with a built-in lifetime), it took months and hundreds of dollars. Now you can't even get parts for it, so what was several thousand dollars new becomes not merely obsolete, but useless.
Many synthesizers out there use custom integrated circuits. Once the production runs for those custom ICs are finished, there just aren't anymore made. Doing another production run later is prohibitively expensive, assuming you can even get access to the schematics or dies (and you can't).
So when/if your favorite synth blows one of these chips, you need to find another chip from another synth of the same ilk to replace it. Assuming such a thing is possible and you're able to find someone to do the work for you. For many old synths with leaky or dying caps and surface-mounted stuff, forget it. It's dead. The sounds are gone forever. You move on to the latest digital whatever it is that doesn't sound the same or work the same.
Entropy is continually whittling away at the number of available "old" electronic hardware instruments. For some, it's not the insides, but the membrane keypads used as cost-cutting measures as a user interface.
And then there's the coming disaster of software synths. At some point, the latest computer operating system will not support your favorite old un-updated vintage softsynth.
I look forward to the ouroboros - Eventually Native Instruments (or their equivalent) will be offering a virtual virtual synth - "Remember the sound of the DiscoDSP Discovery? Check out 'Recovery', which brings it back to life!" - promising to emulate (in software) the software you used to use (which may itself have been an emulation of an actual synth).
In the meantime, don't get too attached to those synth sounds you love. They probably won't be around in 10 years. Almost certainly not in 20.