I think it's okay that the web has different communities on it. The web has grown in population at a staggering rate, if we had the same system we used to have of public boards that would be full to the brim and people would be seeking quieter spots anyway.

I don't like the idea of walled gardens for advertising at all mind you. But different communities, some of them private, makes total sense for such a large population.

No it hasn't. Those walled gardens still comprise a miniscule amount of the actual content on the web. In fact many are link aggregators and depend the existence of content beyond the walled garden to survive.

Nothing has devolved except the willingness of users to explore to find new content. But that's not because it no longer exists, but because for the most part new content is delivered to them.