You're missing the point here. Regardless of the causes of the decline, there are invariably things you can do to improve your content or just generally change your approach.
In fact i might google a spesific product, like a Monitor, and the top result will be "but Monitor XYZ on Amazon". When i click on the link, turns out they don't actually sell or stock that product at all!
With that being said, in any marketing tactic, the right way isn't always the best way - however - understanding why some garbage site ranks for your query is incredibly helpful in figuring out how you can improve your content to do the same.