Based on this
https://www.google.com/intl/en/ipv6/statistics.html#tab=ipv6...
we're probably 15-20 years away from "enough people have IPv6 that you can start dropping IPv4". There's zero chance you could make an IPv5 and have any adoption at all in that time.
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)
Because we're decades in here, and it's still not dominant, for the reasons given.