There are so many websites, apps and media services that are slowly being affected by what I call "information ochlocracy", where those types of organizations which rely on large numbers of people for their existence, will naturally start to target the widest audience/viewership/user base, and the quality of the service as a whole suffer as a result.
Here's the law: "The value of information is inversely proportional to the size of its intended audience." And its correlary: "The wider the audience, the less valuable a piece of information will need to be." These both stem from the basic premise that accurate information is more valuable the more scarce it is.
What this creates basically is a situation in which the quality, depth and timeliness of a piece of information or media service will, over time, always tend towards the most broadly applicable content, despite being less and less specifically useful.
In Google's case we can use cars as an example. Ideally, depending on the exact search term, you might want results providing information about buying them or maybe fixing them. However, maybe more advertising money can be made from using that search term to sell Forza or Gran Turismo video games, so the algorithms will detect this and shift accordingly. And since these games are being talked about on social media a lot, the algorithms pick that up too. Maybe some news mentioned the car or a celebrity bought one. The result might be that you're scrolling through ads from Microsoft and Sony, online shopping results of all the related gaming products, summaries of tweets about it, news snippets, etc., and almost nothing about what you were originally looking for. Searches for historical events like Pearl Harbor could have you scrolling past movie reviews before you finally get to a Wikipedia link.
Ochlocracy is mob rule. It's the evil flip side of democracy, in which people actively make a choice based on considered opinions. In an ochlocracy, the choice is made by the whims of the mob as a whole, and they themselves don't even know the choices they're making. As Google and other companies optimize their services, the numbers will always lead to targeting the largest swath of people and as a result the overall quality naturally suffers.
There's more to this, but this is the general problem. In order to keep growing from now on, every year Google's overall quality will have to suffer in order to be more applicable to larger numbers of people.