I think this is premature. Google is still a huge force in tech. Google search, GMail, DeepMind and more are huge and aren't going anywhere. I have the same experience as PurpleRamen, I truely don't see the decrease in search quality. I think people just expect perfection every time. That's how high the bar is.
Google's biggest problem is that they're a huge force. Google search directs an absurd amount of traffic to where-ever they point their search ranking. The bigger they've gotten the stronger the observer effect has grown.
Search engine spam isn't something that's happened to Google, it's a consequence of their dominance in directing web traffic. No matter which beautiful flower you find, point a power washer at it and all you get is dirt and mud.
I think people’s GoogleFu has also gotten worse. A lot of examples on this page of bad google results have natural language queries.
Google is a traditional search engine. Traditional search engines run on keywords. You enter the keywords you want on the webpage and press “search”. Didn’t find what you were looking for? Add more keywords to make it more specific. Things you don’t want polluting the search results? Exclude a keyword by adding a “-“ in front of it to remove those specific results. Rinse and repeat until you find what you need or give up.