Your idea breaks down immediately when you realize that hosting your own phpBB instance is far outside the capabilities of most users. This means that you're at

Your idea breaks down immediately when you realize that hosting your own phpBB instance is far outside the capabilities of most users. This means that you're at least centralized into groups around phpBB hosts, i.e. this is a federated system. In practice, federated systems become centralized. Email, for example, is now mostly centralized in a few providers: GMail, Yahoo, Outlook, etc. Even if you don't use one of these providers, they have a significant percentage of your emails.