Imagine someone lives beneath a volcano and knows that at some point an eruption will destroy his house such that he no longer extracts economic benefit from it. Do you think that the way he builds his house (e.g. using straw, or stone) will differ based on whether the eruptions come every 10 years or every 100?
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.
And for what it's worth, I consider myself reasonably capable of considering arguments in favour of opinions that differ from my own. On many occasions I have changed my mind about an issue based precisely on arguments that persuasively challenged my assumptions or beliefs. I just have an expectation that those arguments should not be ridiculous.
I can only interpret it as sincere, or very deadpan.
It's a kind of hypersensitivity that acts as quality control. Instead of firing off sarcasm quickly without too much thought and cheaply pointing it out with a "sarcasm" badge, you only get to make jokes or be sarcastic if it is very, very well thought through.
I actually got away with (meaning - positive karma) a joke the other day, although I later pointed it out in a reply to somebody who didn't realize it. Still - felt like an accomplishment and I suppose that's how it should be.