Aaron, how do you figure out these kinds of things (popular searches)? I know there's the Google search keyword tool.
You said, "I would venture a guess that there are no more than a handful of people in the world making more than $100/month."
A handful would be fewer than a dozen, surely? There'd be thousands making more than $100/month. Actually, there'd be thousands making more than $10k/month IMO.
Now, regardless, the majority trying AdSense make bugger all because they're clowns, half-arsed, give up early, don't know what they're doing, etc. Anyone with either some luck or a half-decent plan, some ability and persistence can build up some passive income or at least give it a shot for free.
(I've been making a living off adSense for quite some years now).