That's a great track record for machines doing heavy work! I have a number of vintage machines in my collection, including a 9600 (predates the G3), G3s, G4s, and G5 towers, and they all still work; however, they aren't being used in production work but used for demo purposes. I don't have the liquid-cooled machine that you have, they're rare but I remember they did have a problem with it. Collectors will take it off your hands.
One reason I collect vintage machines is that no other company is blending artistic design with cutting edge technology like Apple. These machines are elegant art, and some are indeed in museums. One of my favorites is the G4 Cube, not as powerful as the towers but strong none the less and a museum favorite, and my Cube still works and is being used to surf the Internet, not bad for a 16-year-old machine. Another favorite is the 20th Anniversary Mac, which is used in my living room for its great sounding CD stereo with subwoofer, radio and TV built in, now 20 years old. These machines do last. I do have some old Windows machines but they're real ugly in comparison, and that has not changed much.
They are really cool, really well-built machines, for sure!
I loved the description of your collection!
Ed