Regarding "decentralization can only exist to circumvent the law", I don't think that in the long term (>5 years out). I touched on this a bit in another comment: https://news.ycombinator.com/item?id=17641098
Regarding privacy, I'm with you there, but I'm very skeptical that very fuzzy privacy concerns can switch enough people away centralized systems with network effects (FB, Twitter, Reddit, YouTube). I think these systems will see adoption from a few groups:
- Privacy advocates and people otherwise interested in the tech by itself - Ostracized groups. For example, see how Voat (not a decentralized system, but an example of a Reddit clone with a ideological mission) wound up as a hub for the alt-right. - The most extreme people trying to boycott platforms like Twitter. For example, people demanding way more censorship from Twitter that are trying to make Mastodon happen because it has content warning stuff and other anti-harassment stuff from what I've heard - Communities that are forced out by the law, like r/DarknetMarkets moving to Dread: https://medium.com/@jbackus/minimum-viable-decentralization-...
So, subcommunities definitely will exit I think but whether they go to other centralized systems depends on UX (which is harder with decentralization) unless a legal reason makes decentralization the only option.