Discrete components are typically easier to replace and at lower cost after several years than much todays totally unstocked, in a few to several short years integrated circuits. If it is running after 40 years of use imagine the bargain and durability with replacing the parts prone to breakdown. In addition, notice how modern companies like Gamut went to point to point wiring for better sonics. However some parts can be hard to find but when you have a legend there are alternatives:
http://www.quadesl.com/schematics.shtml
I would not rely on General Motors being around, at least as well know it, in 10 years but I would rely on a 500C with maintainance updates being around working at mid-century.
I would not commute in a Porsche xxx anymore than I would in a porker SUV but I would experience real driving experience in one (the Porsche that is) and be even more excited and, for me, more rewarded in updating an proven incredibly durable, musical performer like the 500C for another half century of soul connecting bliss. The 500C is not near state of the art like my Hovland but it is a great communicator of music it seems insane how good it is for so little and how much enjoyment can come from it that solid state at many multiples the cost just does not get.