There's still quite a few pockets of the industry where IPv6 just isn't available. Industrial control systems, for instance - there's lots of PLCs and SCADA software that are still IPv4-only. It's changing slowly, but industrial computing runs on thirty year time scales and there's still a lot of stuff out there that won't even run on modern operating systems. We use 32-bit Windows XP VMs or sometimes even FreeDOS for some of the software we use.