If nothing else, I think power consumption is a dealbreaker for a 20-year desktop PC. In (say) five years, there will be an iPhone with the same specs, and then you'll be running a desktop PC for 15 years at 100 times the power consumption for no reason. A good analogy is the way otherwise perfectly-good CRTs disappeared so quickly in favor of relatively expensive and low-contrast flat-panels. I'm using extreme numbers, but at that timescale even small efficiency increases will make it worth upgrading, and efficiency increases are a huge focus right now.
The place to look for a 20-year computer is probably the same place they are now -- embedded systems with minimal power consumption and ample specs for the limited job they're designed for. E.g., cars.
Interesting view which I had not thought of previously in quite that way.
Devil's advocate: My old workstation has a 440w power supply but uses much less than that most of the time. A small percentage of the total electrical power that my house uses. What pressure is there to switch the workstation for a more efficient PC?
The place to look for a 20-year computer is probably the same place they are now -- embedded systems with minimal power consumption and ample specs for the limited job they're designed for. E.g., cars.