It'd be interesting if the law had some sort of "activation" clause, where the copyright would only start to tick toward expiry once a single cent of royalties had been made. That way, things that immediately start to make money (like music and movies) would immediately start to count down, while things that can take years to be licensed (like stock photos) will only start their countdown when someone actually payed for them.

I believe patent expiration varies from industry to industry; certainly copyright expiration could too?