There is ipv6 so you wouldn't use NAT (with few exceptions). IPV4 is a legacy deprecated protocol people keep on using. More specifically, port addeess transl

There is ipv6 so you wouldn't use NAT (with few exceptions). IPV4 is a legacy deprecated protocol people keep on using. More specifically, port addeess translation (what you seem to be talking about when you say 'NAT') would not have have a problem with 16bit port numbers if you can use a large pool of IP addresses for the outside interface(which is almost always the case with natv6). Even with IPv4 it's 65535 sessions per IP, that's a ton of simultaneous sessions. Any router that would do PAT (that I can think) would encounter resource issues at over 20K stateful sessions but if your router can handle it all you have to do is add one more external ip to add capacity for 65535 more sessions. Lastly,you can always do NAT/PAT upstream before it hits the final routers.per access vlan for example or per host if individual hosts need >65535 sessions (adding an ip to the host if that number is greater). Let's say a UDPv2 doubles it to 32bit: for a device pushing 1 million packet/sec (around 1Gbps) that comes out to an added 16Mbps that is pure waste unless someone has a very unique situation where they would push >65535 sessions per IP. There is QUIC for an alternative layer4 as well as a few others. UDP and TCP both do their jobs well,if additional needs arise it would be an entirely new protocol,not an updated version of tcp and udp