IPv6 is over a quarter of a century old at this point.

Yes, but its install base is still less than half of that of IPv4, and plenty of things that claim they support IPv6 don't do it properly. There are countries and industries where it is really entrenched, but in the majority of the networking world, IPv4 is the 100% guaranteed to work solution that all new applications and deployments first target, and IPv6 support is an after-thought at best.

Even Google, one of the champions of IPv6 adoption on the Internet, only started adding IPv6 support to GCP in the 2020s. In all public clouds, there are still numerous services that don't support it at all. And I can 100% guarantee that the next public cloud that anyone creates will start off with IPv4 support; maybe it will have IPv6 as well on day one, maybe not.

> the next public cloud that anyone creates

On the contrary, essentially all of the IPv4 public ranges have now been allocated. The incumbents have bought up what they can, leaving essentially nothing for new clouds to use. It's one type of moat, and the three biggest providers are happy to keep everyone on the far side of it.

A new cloud starting in 2025 onwards would likely be based on an IPv6 native design with IPv4 as the "add on" for public endpoints only, such as load balancers and proxies.

They'll still have 10.0.0.0/16 or 172.16.0.0/16 VPCs as the default, even if they can't buy a large enough chunk of IPv4s to easily give out public access through IPv4 (though they'll likely find some way to partner with some ISP that is switching massively to CGNAT and can free up most of the IPs they would normally give to their clients, or something).

IPv4 is far too ingrained to start with IPv6 only.

As far back as 2015 I tried to build an all-IPv6 network for a lab environment. It very nearly worked and the few remaining issues would have been quick fixes for vendors.

We’re closer than most people assume.

I'm not saying it can't work, I'm saying it's no one's first choice, even today. It's just extra risk, less usable (10.0.123.234 vs fd00:ec2::1234:5678), extra hassle (multiple IPs per NIC, extra hoops to access Github, etc).