A lot of the reason VMS is still around is that there isn't really that easy of a migration path away from it.
When Windows came out, you could still (and can still) run most DOS programs on it.
I can't run Prairie Dog Hunter on my XP box...I sure do miss that game...
On the other hand, when you've got rock solid hardware and software, why migrate away from it, especially when you need to ensure 100% uptime.
A client of mine did ALL of their real-time financial transactions on a VMS cluster. They were a major clearing house, and downtime would have cost them in the hundreds of thousands of dollars an hour!
Another client, a manufacturer, uses it for their production lines. They're a JIT (Just In Time) supplier for an auto manufacturer, and they've got a fixed amount of time to supply the auto factory with the components ordered, othetwise, it causes the auto plant's production line to shut down. If they're late, they get "fined" a huge amount of money, for every piece that's late.
Mark