IPv6 celebrates its 20th birthday by reaching 10 percent deployment (Ars Technica)

That's why the comment of mine you're replying to deliberately mentioned TCP, "IP has options, as has TCP." and wasn't specific on at which level you'd use the option, "You can include the expanded address space identifiers (src and dst) as an option.". ;)

At this stage, IPv6 hopefully can't fail. However, as per another comment, adoption of v6 is still sufficiently slow that if another, more useful (e.g. better transition) protocol came along that then people might favour adopting that. Unlikely at this stage, but not impossible.

Such a protocol might even go so further than the above, and do the extension at the HTTP(S) level. E.g. I can SSH into hosts that do not have any public IP addresses and are even behind firewalls, by making such hosts connect to Tor and publish a tor service on a Tor address. And note that this is possible directly with clients that support the SOCKS API to offload a lot of the networking details to the local Tor relay itself - no need to update the clients to deal with a new address family and protocol. Even non-SOCKS clients potentially still can access Tor services, if they're fairly standard in what they do network wise, using an LD_PRELOADED library to capture regular network API uses and divert to SOCKS.

And note that the host with Tor need not have any direct connection to the IPv4 Internet at all. All it needs is to be able to access a HTTP(S) proxy that has public Internet access. That host can then access the Internet, and other Tor-enabled hosts can access it.

See also LISP, which can also run v6 LISPed space tunnelled over v4 I think.