The Laffer curve deals with output from labor. When you tax someone you reduce the money they get per unit work, i.e. their wage. If the wage is too low they might not find it worth doing the work. If I'm a paid shill/some job and I'm taxed 99%, I might stop shilling because the effort outweighs my salary.
Regarding automation, see the lump of labour fallacy. Humans will just switch to tasks that aren't automated (this has always been happening and it's happening now). If humans are not needed at all, then we've reached utopia/post-scarcity society.
Humans don't work less because of the hedonistic treadmill. For example, most HNers have enough food and other things to survive which might be considered paradise by people 500 years ago (famines were common back then). Historically, poverty often meant death from starvation.
Our standards of living have been constantly going up, so even though there are productivity gains we end up working more. Of course, that's a voluntary choice so I don't see anything wrong with that.