Primecoin also ultimately had limited utility. It found the world's largest prime number thousands of times, continually beating it's own record. At some point

Primecoin also ultimately had limited utility. It found the world's largest prime number thousands of times, continually beating it's own record. At some point we had more prime numbers than had mathematical utility. My unprovable guess is that any other non-sellable problem will ultimately have the same outcome. It's interesting and useful for a bit, but when you have a billion dollars of hardware grinding nonstop for years in a row, you eventually exhaust everything interesting about the original problem and then it's back to the same waste you started with.