[Denatonium]

This is true, but the beauty of UDP is that it's basically just a raw socket with a tiny 8 byte header slapped on top, with 2 bytes for source port, 2 bytes for destination port, 2 bytes for length, and 2 bytes for a checksum.

You could slap a UDP header on top of the TCP header and get the benefits of TCP with the hole-punching capabilities of UDP, provided you implemented some kind of keep-alive functionality and an out-of-band way of telling the "server" to establish an outbound connection with the "client". Or use QUIC, assuming it fits the use case.