I don’t think that is completely truly possible that way. In the end there are a bottle necks - for example the peering routes. It would be insane to have them scaled for the worst case of all potential users being online at the same time and actually using their bandwith 100%. Which in turn likely means that providers are “overselling” what they have. If all end-users would actually use the 100% of their bandwith 100% of the time, the providers would be fucked.
TURN is only used as a relay. It’s just a low(er)level transport layer, not specific to VoIP. Typically it is used as fallback when P2P communication between the attendees is not possible (for example due to a restrictive firewall).