Correct me if I'm wrong, but if you have the storage solved for the rest why not just use that same storage mechanism? If you're using local storage on your legacy applications and are essentially bound to the one node why couldn't you tag one node and use hostpath in kubernetes?
Since these applications are fundamentally not designed for something like kubernetes what everyone seems to be asking for is Kubernetes to provide network storage that has the good things from NFS but faster. I could be way off base here but asking for a simple and performant solution to this problem from projects like kubernetes just doesn't seem reasonable. Network storage, particularly the type that provides traditional filesystem semantics, is a hard space all on its own.
To put it another way, it sounds like people are asking kubernetes to take single node applications dependent on traditional filesystems and provide a magical performant network storage layer to make those failover seamlessly between kubernetes nodes with no (or tunable?) data loss. For those asking for this how would _you_ go about creating such a system? Just thinking through that a little should make it clear that what you're asking for isn't just a solution to a very difficult problem, it's a solution that is, I think, worth quite a lot of money to the person/group that solves it elegantly.
I would genuinely love to be proven wrong here so by all means destroy my argument with extreme prejudice.