Comcast's entire core network is pure IPv6. Every cable box, cable modem, everything is connected and managed using IPv6 addressing.

None of it is IPv4.

T-Mobile's core network is entirely IPv6, IPv4 lives on the edge only.

Facebook's entire internal network is IPv6, they have IPv4 edges that translate to IPv6 internally so that it is routed as if it were IPv6 and all services see IPv6 only.

Sorry, but your "adoption is low" is very VERY wrong and IPv6 has already solved a lot of problems, for example the "we are out RFC1918 space, and adding more NAT is not the solution".

That last part is literally why Comcast moved to an IPv6 core.

Mobile networks, for example, have huge deployments of IPv6. Whether or not they are used to access IPv6-only services doesn’t change the fact that IPv6 is an integral part of their network design and therefore an alternative to IPv6 would need a migration pathway from it.