Since the IETF saw that there was gonna be an industry-wide overhaul in any case, it did this complete overhaul, tossing in everything learnt in the years of IPv4, so that another IP transition won't be likely in the next 50 years, if ever.
By this point, even the luminaries at the IETF have realized that the design for IPv6 as a replacement for IPv4 is fatally flawed. How flawed? Flawed enough that there is a high probability that a worldwide transition to IPv6 will never actually happen.
Now sure, there are technical advantages to a clean slate design, but a clean slate design is also unfortunately almost useless as a replacement for IPv4 in the real world. There is no incremental advantage and extraordinarily high costs to adding a separate numbering plan to an existing network, so no cost conscious organization ever does it unless they are forced to, and probably never will.
At this point I would lay odds on an IPv7 eventually being developed that is a revision of IPv6 with the incorporation of the IPv4 address space in a routeable fashion, and which assigns each IPv4 address a network prefix that an entire subnet of devices may eventually be directly addressed behind, in addition to the default.
Why? Because doing anything else would be one of the biggest wastes of resources the world has ever seen.
Any downsides? An IPv7 router would have bigger routing tables than an IPv6 only router, but the routing tables could be used to route IPv4 packets, and as it is not likely IPv4 is going away anytime soon, the same overhead is there one way or another.
A wide scale deployment of IPv7 would require hardware upgrades in some cases, but for most people it could be deployed silently, without them ever needing to know or care. A simple software update would be all that was necessary, and a few years down the road nearly all IPv4 capable devices would handle the expanded address space in a usable fashion without any renumbering or other configuration changes. That would save billions of dollars a year in unnecessary administration costs worldwide.