Now you're counting DNS servers. Because the provisioning of IPv6 DNS addresses has been such a mess and still is problematic, many dual stack systems do this over IPv4. And the DNS servers they talk to may be IPv4-only, or IPv4-only users may talk to dual stack DNS servers.
Which is why I suggested trying it on ONE service and watching it closely. What I have done is selected a "best candidate" for a test. I am not implying that "this is guaranteed to work".
If the results of world IPv6 day are as we expect and only 0.1 - 0.2 % or less of all people have problems, I think the best way forward would be to have a second world IPv6 day where we again enable IPv6 industry- wide but this time we don't turn it off again.
0.1% of users is a HUGE number if you have 1,000,000 subscribers. Are you prepared to field 1,000 helpdesk calls or lose 1,000 customers? Now imagine 100,000,000 subscribers. Are you ready for 10,000 support calls or the loss of 10,000 paying customers? It isn't something you just throw out there on a whim and tell people to like it or lump it if there are potentially a lot of people involved.