Conversely it means people who have ISPs that do IPv6 just have IPv6 and don't need to turn it off. Because it just works. The other day my IPv4 was down and I didn't even notice.

I don't expect any ISP to do IPv6 today and deploy routers with a flow label bug... Those types of bugs no longer go unnoticed.

IPv6-only ISPs might hit other issues, though. They have to bridge to IPv4 somewhere.