> It failed to solve the problem of impending IP address depletion

I wouldn't say so. Some mobile carriers and big data centers have used IPv6 to pretty much completely solve the problem of being able to assign a unique address to devices.

For mobile devices, moving 50% of traffic over to IPv6 means buying half as many CGNAT/v6-to-v4 boxes (of various kinds).

And on the v6-inside, unique address can be assigned. Legal requirement and court orders suck when you get "who had A.A.A.A:32800 at time T?" if you have to go through three levels of NAT to decode that. So even if a customer only accesses IPv4, having their actual handset only be assigned IPv6 makes things easier and cheaper. Even if they share an outside address, there's only one translation so the inside is unique.

For big data companies, it means not needing to solve the problem of running out of 10/8 (yes I'm aware of the other private addresses), and having an address plan problem any time they make an acquisition.

And I've seen large providers who build their whole actual network with IPv6, and only tunnel IPv4 on top of it. Huge savings in complexity and cost of IPv4 addresses.

So what I'm saying is that I've seen first hand in multiple large providers of different kinds how IPv6 is delivering incremental payoff for incremental adoption.

It doesn't have to be 100% before we get ROI.

> it is not a success.

About half of even public traffic on the most complex and distributed system ever built is IPv6.

It's going slower than I'd like, but it's definitely paying off.

There are still ATM and X.25 networks out there, so is IPv4 a failure? (admittedly, a bit hyperbolic)

I'm working on a problem right now at a large company to move a thing from IPv4 to IPv6 because the existing IPv4 solution is running out of addresses, and it's impossible (for multiple reasons) to "just add more IPv4". Can't go into details, sorry.