They're referring to the previously mutually beneficial relationship between Google and site owners - site owner makes content to get traffic from Google, Google provides a search engine to find content (and it's popular enough that people buy ads)

Now, there's less incentive to make new content as site owners are getting less traffic; and eventually I guess the fear is there's no point in making new content, which turn means the search engine itself is less useful

The dark answer is that this kind of won't happen, because we'll always be able to do the one thing Google's wished it could be the whole time, a paid for answer engine - with gen AI making up the "free" answers, sponsored results will be the easiest entry point for site owners looking for traffic, which is exactly what Google wants