First, I didn't say they shouldn't expire, just that it makes less sense for them to do so.

A patent expires and someone else can produce the same thing cheaper, often much cheaper. It prevents a monopoly and benefits the consumer. A copyright expires on a book. So what, someone else writes the exact same book word for word? It prevents a monopoly sure, but where is the benefit to the consumer/public? How much cheaper is a publisher going to sell that book for? One dollar, two, five? The minimal benefit to the public doesn't justify forced expiration of a copyright, in my opinion. Now, if the original author or his/her beneficiaries try to charge some outragous amount for the book, I can see a reason for the copyright to expire. But frankly, most people just won't buy at that price.