The Case For Perpetual Copyright
copyright law exists to give incentive to artists to create socially and culturally significant works. perpetual copyright gives no such incentive, instead it provides incentive to create works intended to do nothing but generate profit.
And when something can generate profit, the tendancy is to take it to the extreme. I could see corporations copyrighting everything, even down to short sentences, in perpetuity. It would be a land grab. This has already happened with patents. You can't write a computer program anymore without using algorithms that someone has been issued a patent for, even though you didn't even know their patent existed.