I don't see why copyright terms need to be empirically justified. Land is passed down from generation to generation, and few people ask: "what's the empirically justifiable duration of land rights?" If we're going to have inheritance of property,[1] I see no reason to extend stronger protections to dirt peoples' ancestors stole from native Americans than to creative works they brought into the universe through their own ingenuity.
[1] Which is a controversial issue, but a separate one.
[1] Which is a controversial issue, but a separate one.