"Unfortunately, no matter how hard you try, there is a certain percentage of nodes for whom hole punching will never work."

It might be helpful to cite the percentage

It's relatively small

A default policy that relays traffic through a third party is asinine

For the small percentage, the third parties will always be there if they need them. The internet has an enormous supply of middlemen, like Google

For everyone else, the third parties, i.e. the middlemen, can be avoided