I don't have any hard numbers, but I have heard ~85% ICE success rate with out TURN. But you are right, in some cases WebRTC will fail without TURN. Just no one wants to pay to run those servers :)
I would love to see TCP hole punching in ICE, but it sounds like it is super hard to get right.
Consumer internet does a lot better, lots of those failures come from Government/Military/Medical I bet.
You always need a at least a STUN server and in my experience
that 85% isn't remotely true. For example STUN-only never worked
from mobile internet (only tested some german providers).
I would love to see TCP hole punching in ICE, but it sounds like it is super hard to get right.
Consumer internet does a lot better, lots of those failures come from Government/Military/Medical I bet.