Google is a self playing piano with clueless leadership. There is probably no plan involved.

Just managers doing what they get more money for or devs hunting promotions by increasing ad revenue by 0.01% in the short term one sting at a time.

the way you phrase it there makes it sounds miniscule, but you scale that up to the size of the SEOified internet and the numbers are surely into the billions

I was thinking the same. Taking in consideration the vast amount of such SEO farms, there's surely a lot of ad money to be spend/earned if you prioritize the "right" sites.

Last time I saw, Google gets much more revenue from ads on search than from the entire 3rd party ecosystem.

a big number that's much smaller than a big number is still a big number

I don't think it's an intentional decision anyone has taken, or that they intentionally made the search engine the way it works now, but more of a "there's nothing wrong here from out perspective, so what's there to fix?" kind of thing.