[SEMW]

> If artists are motivated by the economic incentive afforded by copyright, then the level of that motivation is variable based on the length of copyright.

While that's undeniably true, I think the objection is to your leap from observing that motivation significantly increases with term length at term = 1 day, to concluding it's still significantly increasing at term = life+70yrs.

Economic incentive can be measured and modelled, and while there isn't that much research into this, there is some. One Cambridge study (Pollock [2007]) found that the optimum length of copyright term is ~14 years (any longer and the economic benefits of works going into the public domain outweigh further increases in incentive to produce). Now, yes, economics models should be taken with a large grain of salt (especially ones when it's unfeasible to test their predictions). But it's a start, and certainly there don't seem to be any econ studies supporting a length anywhere near life+70 or longer.