[fragmede]

> middle boxes won't speak the protocol

Middle boxes only need to speak IP, which they do already, and not block nyxpsi packets (which they probably do).

[zamadatix]

You'll still run into trouble with anything wanting to allow everything but trying to do NAT. I'd hazard to guess this actually still uses UDP under the covers for that reason (but haven't bothered to verify). QUIC and HTTP/3 went that route for the same reason.

Edit: It does.

[wyager]

There's not really a sufficiently compelling reason to use anything except UDP for future protocols aiming to replace TCP