"When a measure becomes a target, it ceases to be a good measure" (Goodhart's Law).
Google measures how "good" pages are for a given search query. (It doesn't matter how.) Those who own the pages want those pages listed as high as possible, so the measure of how good their page is becomes a target (for them, not for Google). That's enough to make Goodhart's Law apply to Google rankings.
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It's not a summary of what happened to google (as far as I can tell), but an example of what it's become.
I want to know what happened. What were the individual things that made sense in isolation, but added up to this steaming turd.
"When a measure becomes a target, it ceases to be a good measure" (Goodhart's Law).
Google measures how "good" pages are for a given search query. (It doesn't matter how.) Those who own the pages want those pages listed as high as possible, so the measure of how good their page is becomes a target (for them, not for Google). That's enough to make Goodhart's Law apply to Google rankings.
Too much focus on ad revenue to drive revenue growth to the detriment of the user search experience is my guess