I agree with you on point 1; if Google's search quality is not the best than people will (eventually) go elsewhere.
I disagree on point 2. Users on low quality AdSense sites almost certainly arrived there from a search engine so if I can display my adverts on a users landing page it will be almost as good as if they arrived on my site straight from Google.
In fact, AdSense ads on sites with shitty content are even more likely to be effective, because users won't find what they're looking for in the page content.
Bingo. When our indie music site's stream server would go down, our adsense ctrs would skyrocket. The people who come from google go apeshit and start clicking anything--especially adsense units--when the content they were expecting isn't found or isn't functional.
I disagree on point 2. Users on low quality AdSense sites almost certainly arrived there from a search engine so if I can display my adverts on a users landing page it will be almost as good as if they arrived on my site straight from Google.