I think it would be interesting to think about centralized systems in terms of agency and bundling(losing choice).

Spam, moderation and deception to me is another problem that needs to be dealt with. I'm using spam in the general sense of things off topic. If it's not dealt with at the system level then it will need to be dealt with in a higher level.

Who pays for the service? facebook wouldn't even be a thing if users wanted to foot the bill for a server(or service).

Also, at a society level a completely private, untraceable money/communication system scares the living hell out of me for the criminal potential.

Although there are uses against authoritarian governments(and/or over reaching rules), there is a lot of criminal activities that are immoral and flat out repulsive as well that such a system will enable.

"There is a mountain of money to be made by turning users into captives and farming them like domesticated animals;"