Google certainly had to update their algorithms to cope with SEO, but that's not why their results have become so poor in the last five years or so. They made a conscious decision to prioritize profit over search quality. This came out in internal emails that were published as part of discovery for one of the antitrust suits.

To reiterate: Google search results are shit because shit ad-laden results make them more money in the short term.

That's it. And it's sad that so many people continue to give them the benefit of the doubt when there is no doubt.