You would start from scratch to develop chips that would support your new formats. IPv4+TCP and IPv6+TCP is literally baked into hard silicon on many, many dev

You would start from scratch to develop chips that would support your new formats. IPv4+TCP and IPv6+TCP is literally baked into hard silicon on many, many devices, even if not for all phases of the protocol. We're talking redesigning switching chips, redesigning TCAM (content-addressable memories used to cache routes), etc. on top of designing yet another protocol. At least old IPv9 had the benefit of some level of support in sw/hw when it was proposed (as it reused CLNS from OSI for internetwork protocol)