Perhaps the centralization is a necessary function of technological progress. Once Web 2.0 was a thing, everybody had comment sections. Now the dial is being turned back, and comments sections are beginning to be seen as tacky.

In some ways, the web is already decentralizing, or at least moving away from monolithic control by social networks. For younger people, Facebook is basically dead and snapchat does the job.

I doubt we will see significant technological innovation in the next 20 years that matches the scale of interactivity the web brought for the first 20. Maybe VR will be that next step, but even that won't likely change the dynamics of casual web browsing in between work, for example.

So in short, I think that since we've developed all the basics, the state of the web will start to resemble fashion, rather than technological progress.