You used to be able to have a small program written in BASIC or whatever that would give you a simple popup alert on desktop. Now you need to worry about backwards compatibility, different OSes, etc. But guess what? You can still write simple programs, without all the complex stuff, if you want to. The exact same thing applies for web. You can bundle megabytes of javascript, for some fancy stuff, or you can have simple static index.html with just text.
"The web was never supposed to be a few walled gardens of concentrated content owned by Facebook, YouTube, Twitter, and a few other major publishers. It was supposed to be a cacophony of different sites and voices."
It still is. Facebook, YT and Twitter are simply more known than your local pet owners forum but that doesn't mean the pet owner forum doesn't exists. It's similar to supermarket chains and your local grocery store. Both still exist.