How many books written today would not be written at all if copyright lasted, say, 20 years? 10? "life of the author" is far too long; all culture is built atop

How many books written today would not be written at all if copyright lasted, say, 20 years? 10? "life of the author" is far too long; all culture is built atop other culture. Copyright is a limited exclusive grant in exchange for the expectation of having more works written/created/painted/drawn/designed. We should not just ratchet up the duration or scope of that exclusive grant without the expectation that we'll get more value out of it (more works) in the trade.