Why would we believe that the incentive is materially different if copyright is 50 or 30 years rather than 100? The vast majority of earnings on a creative work are in the few decades immediately following release. Who is going "I won't write this book because it might make me money for only 30 years rather than 100"?
Historically there might be just as many authors (and other artists) who’s works didn’t become popular and make money until after their death, as those that became rich and famous during their lifetime.
Of course it seems “times are a changin” and the vast majority of fame comes in the form of a viral and fleeting 15 minutes. Perhaps the law could catch up with the times, by having life imitate art and literally give copyright protection no more than 15 minutes.
