Take a site about widgets that used to get ads from widget sellers, widget makers, etc. Now, I visit that site I'm more likely to see some kind of less relevant local ad or a retargeting ad from some site I've visited recently.
I presume that Google wants to make money as much as the site owner and would attempt to deliver ads that maximize revenue, but do you think that's the case?
Example: Lately, I've been seeing a lot of ads across many sites for a web hosting company whose site I've visited lately. (These are clearly retargeted, as hosting is wildly off-topic for the sites.) I'm a customer of that firm, I don't need to click on their ad to learn about them, and (in short) those ads are a complete waste for the site owners.
Now I assume that I'm the exception, and that spread over hundreds of surfers who visited that host's site the ads WILL generate profitable clicks. But somehow I think the old widget ads would do better.