[ay]

Once the critical mass is reachable over IPv6, the rest becomes "long-tail" which will eventually be too expensive to bother about.

An extreme example: if an internet user has IPv6 and just uses gmail, youtube and facebook - they can turn off IPv4 right now and not notice anything.

Lee Howard has had an interesting presentation on the subject: https://www.nanog.org/sites/default/files/wed.general.howard...

p.s. Quite a few million of T-mobile's subscribers are also IPv6-only today, just that they use NAT64/464XLAT to connect to IPv4-only services.