I think the OP is weak because it conflates ads and seo spam. Yes, Google went all in with ads, and yes, this hurt its credibility and the quality of its products.

But there is no conceivable universe where seo spam isn't the arch enemy of Google. Google needs to fight spam to survive, it knows it and it does. But that's hard. So hard in fact, that nobody else has cracked the problem, and for all the anecdotal evidence of someone switching to Bing, Google's marketshare is still utterly dominant.

The Dead Internet Theory may have some weight: Google hasn't dropped the ball, but it is slowly drowning into the sea of "content-free content".

But if that's the case, so is everyone else.