> Some part of us instinctively understands that happiness isn't the point and can itself be counterproductive because we need some amount of unhappiness to stay motivated to do what we are actually here to do, which isn't be happy.

I don't think they are saying we're stuck in some endless loop to specifically acquire money or stuff, but that we are stuck in an endless loop to do something, whatever it may be. This could mean acquiring stuff for some, volunteering and donating for some, baking pastries or going fishing for others.

Decline in fertility rates can be attributed to external causes, not because humans fundamentally no longer want to procreate.