Human beings are relative people. Everything we do, everything we feel, everything we think, is relative to other things we do, feel, or think. Your perception of hot and cold is relative to the most recent thing you felt. The Exploratorium has an excellent demonstration of this. They have a heated pipe, a cooled pipe, and a pipe at room temperature. You grab the heated with your left hand, the cooled with your right, and then the pipe at room temperature. It feels cool with your left hand, but warm with your right hand even though it's the same temperature. You cannot absorb a new concept unless you have a way to relate it to other things you know.
In order for new creative things to have any meaning, they must relate in some way to existing things. Rap artists explicitly include snippets of other music in their works. Musicians use the twelve-tone system, not because it is intrinsically correct, but because it is familiar. Non-western cultures use microtones in-between the twelve, and they handle that just fine.
In order for new things to have value, they must be built on top of the old things. If copyright were perpetual (and currently nothing has gone out of copyright in Mickey Mouse's entire lifetime -- and he's already pushing the average lifespan of an adult male), then the value of old things would be going up, up, up. This reduces the incentive to create new things, because the new things create demand for the old things, which decreases the available compensation to pay for the new things.
Science Fiction authors have explored the dynamics of perpetual life, and there seems to be a consensus that it is mortality which drives the creative process: create or die, because otherwise you'll just die.
Copyright must expire, just like people must die.