Actually, I'm mostly basing this on SSB, since it is the only one of these three I've used seriously, and I've followed its development since when it was just D

Actually, I'm mostly basing this on SSB, since it is the only one of these three I've used seriously, and I've followed its development since when it was just Dominic Tarr and Paul Frazee in an IRC chat. The system has gotten a lot more complex since the early days, and although I no longer know all the internals, it seems that the pub servers are pretty integral to its operation, and you can't get someone onto the system without one. I agree that an SSB-like system with pub servers removed and replaced by STUN/TURN servers which would only facilitate WebRTC would not have this problem. Just to be clear, the reason for this is that STUN/TURN servers are so lightweight and have very little to do with the actual operation of the system, so they could be put on a list on the main website and supported by very small donations. Then again, like you said, one could describe such a system as "decentralized", not "federated". I also suspect it would have performance characteristics that many users would not like.