I don't know how to get any good data on this, but I think saying "Google's bread is buttered by <undesirables> ..." is at least accidentally misleading. Google supports a tremendously diverse ad ecosystem, some of which is questionable. However, at least in my experience, very few of my search queries involve such ads. The perception (if not provable fact) that they're mostly a class act is a big part of their enduring brand value.

Of course, it's possible that the "typical" user gets a much larger fraction of "scam ads" than I do. I don't know how to evaluate this empirically without knowing something about the overall query distribution.

Still, at least for my search patterns, there is a very large difference between AdWords and "scam filled advertising". This is in sharp distinction to my average experience on, say, Facebook (never mind other fairly mainstream web behaviors like gaming forums).