I think the open Internet is often said to have begun declining with the rise of walled gardens, notably Facebook, 2010 or so. A further factor was Google's degeneration from relatively fair broker to (biased) "question answerer", with a notable step 2019-2020 but with earlier steps also.

Google was using it's version of AI to push "answers to questions" a while before ChatGPT, these just weren't very good so they weren't sorted first.

But it's hard to really say - as long as the Internet has existed, people have been crafting schemes to control/manipulate it and by now those schemes have tremendous weight.

The main thing is the early Internet was a unique because it's success was an accident. Corporations and bureaucrats imagined the "Information Superhighway" as an enriched cable TV and this made it such a failure that amateur use of the Internet wound accidentally "going viral" and only over time did all the crap but it's been a while and the Internet is close to a "super-duper-information superhighway" today.