As far as the sound is concerned, I'll attribute the decline to significantly decrease competition and demand (like many of you have already indicated).
For the build quality, durability, and lifespan, I think it's the shift to a "disposable" society. Cars, lower-end receivers, washers and dryers, telephones, televisions, mass-produced furniture, and so much more just don't last as long as they used to. That's not to say that they're not functionally better in many ways; they just don't last as long. I've had a cheap gas stove from the 50's, and a cheap gas stove from the mid 90's, and while the newer stove is safer (avoiding pilot light issues), the older stove might still be functional in another 60 years while the new one might crap out tomorrow. It should be no surprise that vacuum tubes (no longer built in the UK, Germany, or North America) also have shorter lifespans.
For the build quality, durability, and lifespan, I think it's the shift to a "disposable" society. Cars, lower-end receivers, washers and dryers, telephones, televisions, mass-produced furniture, and so much more just don't last as long as they used to. That's not to say that they're not functionally better in many ways; they just don't last as long. I've had a cheap gas stove from the 50's, and a cheap gas stove from the mid 90's, and while the newer stove is safer (avoiding pilot light issues), the older stove might still be functional in another 60 years while the new one might crap out tomorrow. It should be no surprise that vacuum tubes (no longer built in the UK, Germany, or North America) also have shorter lifespans.