Especially if you were predicting a migration across any compatibility break would take decades or more (or potentially forever live side-by-side), there's very pragmatic reason "rip the bandaid off" and to change everything all at once rather than make a series of far more painful "smaller" migrations that would each individually take nearly as long.
That exists, it's called enhanced IP or something. I'm having trouble finding it now since it's been a few years, but I've had a customer using it. Of course nothing besides the endpoints understood it, so it had to be wrapped in UDP. Horrible.
The only major change is that instead of the smallest routeable unit being a single address the smallest unit is now a whole 64 bit subnet so hosts have a bunch of bits they can use for various purposes. Network administrators don't have to worry about that for the most part, the hosts figure that out on their own.
I guess there might be a security implication if someone has built their security system around knowing everything on the network via DHCP. But that was always a mistake. If you want real security you need to use 802.1x or similar. Being told you can't use DHCP to secure your network anymore is probably shock to some admins, but hopefully they take the opportunity to reconsider their system from the ground up.
I do use ipv6 at home, and at some of our customers, but some of them would have to essentially redesign their entire networking/vpn/security plan (and yes, existing ones are not good, split horizon DNS is almost always present, DHCP is not only used for security, but also for provisioning, there are products that scan subnets and provide reports on devices required for various compliances ...). And that is a hard sell, for what they see as next to no benefits.
Because we're decades in here, and it's still not dominant, for the reasons given.
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.
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)
The misconception is that IPv4 could have been expanded in a compatible way. That's not GP's point though as I read it. They could have made an incompatible IPv5 which was exactly like IPv4, except with additional address bits.
Then the operational side would be similar, and you wouldn't have to learn everything over again.
Instead they changed just about everything.