Copyright is an artificial monopoly set in place to guarantee that artists get a piece of the cake from distributors. The duration of this monopoly is completely arbitrary, and ideally it should be "long enough to make art creation a viable trade".

ideally it should be "long enough to make art creation a viable trade"

And, IMO, 10 years is in the ballpark for that to be true. That's ~5 major pieces of art as a minimum for a popular artist to have a career (assuming their 20s through 60s) [assuming each protected piece can sustain them for a decade].