Don't forget that like many things (cars being the best example), planes are also consumable. They are consumed by use, (certainly misuse) and time as well. As something is consumed, it's value will decrease. As the issues with aging aircraft come more to the forefront, the value of these aircraft will be further affected.
You can pour a lot of money into a restoration, and have a desirable aircraft, but that's not a way to get a deal in aircraft ownership. Certain types have traumatized parts support, so even a good deal could become horrible if you break or wear a part which is not replaceable. The aircraft manufacturers of decades ago probably did not imagine that their aircraft of that era would remain in service so long, but they did.
But an informed purchaser will know what to consider before purchasing, so sellers may be pricing down to allow for what they might find - or already know is there. I have flown a lot of planes which I loved, but would never want to own, as I know what it will cost to keep that plane in service as it ages.