[nknighthb]

Dual-stack, proxies, and (if absolutely necessary) actual tunnels can compensate for critical IPv4 apps. You can layer whatever you need to on top of IPv6.

Non-critical applications where people aren't willing to jump through any hoops... Well, it's not exactly news that sometimes old apps get left behind in technology transitions.

[sp332]

Dual-stack still requires an IPv4 address.

[nknighthb]

Of course it does, but not necessarily a publicly-routable one. Dual-stack is necessary because otherwise the IPv4 application literally won't run at all. How the packets actually traverse the public internet is a different matter addressed by things like NAT, proxies, or tunnels.