I'm guessing that even in a future in which we have 100% of IPv6 deployment, we would still run our loopback/LAN interfaces with IPv4 for simplicity...
It's too late. I have already run into issues on Linux distros with some daemons binding to 127.0.0.1, but localhost resolving to ::1.
There is only one local IPv6 address:
::1/128
Unlike IPv4 there is not an entire /8.
IPv6 still has the whole IPv4-mapped range at ::ffff:127.0.0.0/104