I don't see why it can't be both. The web has less good content AND Google has altered their search results such that results are worse generally for us, but better for them and their bottom line.
But if I'm wrong, that would be great! The comment that started this thread made me think that it seems like I'm not wrong though.
And I don't even think this is the primary issue with the web sucking. The web is just an entirely different thing than it was during the period we're all nostalgic for. I used to search for information and find it on independent pages that I never would have found without a search engine. But now that same information I used to search for is all on wikipedia, with much better citations than I used to get.
And pretty much everything other use case of search is now behind some social media walled garden.
IMO the web was kind of just a victim of its own success. There stopped being much incentive to put new things on the web on a huge proliferation of independent sites.
(NB. This is terrible for Google! If my take on this is right, then their business model is going to eventually be in real trouble. This is another reason I'm pretty skeptical that they have caused this to happen. It's bad for them, I don't think they're happy about it at all.)