Comment by toast0

Yeah, so... You can do it. But only for some values of you. In a NAT world, the NAT needs to understand the protocol so that it can adjust the core multiplexing in order to adjust addresses. A best effort NAT could let one internal IP at a time connect to each external IP on an unknown protocol, but that wouldn't work for QUIC: Google expects multiple clients behind a NAT to connect to its service IPs. It can often works for IP tunneling protocols where at most one connection to an external IP isn't super restrictive. But even then, many NATs won't pass unknown IP protocols at all.

Most firewalls will drop unknown IP protocols. Many will drop a lot of TCP; some drop almost all UDP. This is why so much stuff runs over tcp ports 80 and 443; it's almost always open. QUIC/HTTP/3 encourages opening of udp/443, so it's a good port to run unrelated things over too.

Also, given that SCTP had similar goals to QUIC and never got much deployment or support in OSes and NATs and firewalls and etc. It's a clear win to just use UDP and get something that will just work on a large portion of networks.