> But really, these methods are needed for traversing between any two networks behind stateful firewalls, which will pose a barrier to P2P indefinitely.

That's true. The actual problem are symmetric NATs where every peer sees a different port number. This makes traditional NAT-traversal impossible and you have to resort to port guessing/scanning. See for example https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&d...