To me, those are fun things to solve.
The answer is to still have clients and servers, just that the servers deal with the grunt work of routing traffic and the clients just stream the data through a server.
Push vs pull, depending if you can get upnp to open a port to the client.
Skype was originally a peer to peer application, seemed pretty popular and while it still was p2p? It was written by the people who wrote Kazaa, remember that?
Sure, but Skype adopted a peer-to-peer design back in 2005 or so when expectations were a lot lower - no smartphones, no group calls, no 1080p, no cross-platform, and if it's unreliable or needs some fiddling around to get it to work properly, such is life - because the competition was paying $$$ for an international phone call that wouldn't have any video.
These days if you want to compete with Skype and Google Hangouts and Discord, I expect my regular four-way video call between people using Windows, OS X, Ubuntu and ChromeOS to establish first-time with clear audio and video to all users.
Hence why VoIP like WebRTC encourages direct media rather than wasting a trip to a server, the server's peering can often be much worse than a path between two users on residential ISPs.
https://arstechnica.com/information-technology/2016/07/skype...
All of the performance improvements could have been done while maintaining the core of P2P, the ONLY reason to centralize it is to make it easy to tap, and harvest data from