There is an aspect of cannibalization, Google can't really grow much more than they already have in the search niche and most other things they've tried (cloud, streaming video, social media, shopping, music) has competent competitors. The only way to present black numbers to the investors is to essentially milk the cow harder in a way that makes their offering worse.
I do agree that what has allowed Google to stick around is indeed that they're very well rounded. There are many competitors that do one or a few things better, but there isn't really a compelling replacement.
My point though is that most of Google's problems with "SEO" stem from their monopolistic position in the market. Their search engine is essentially a firehose of cash flowing into whatever ranks well. This creates a sort of dynamic where what ranks well gets more money, and is able to proliferate; and what ranks poorly dies off. This is essentially an evolutionary process. What is perceived as adversarial SEO is essentially Google's own shadow. It takes that shape it does because of the things they value.
Search engine spam is not just a flaw in Google's algorithms, it is a consequence of their position in the market.
Whatever replaces google, whether it's some sort of AI or Marginalia Search (not very likely) will have exactly the same problem as long as it exerts the same dominance over the web's traffic.