Well, it's true that one can do just about anything with procedures that IBM has traditionally done with open, read, write, etc. But IBM's interface is more familiar, streamlined, and consistent.
It appears to me that Open I/O could signal a new level of cooperation between IBM and 3rd party vendors to extend RPG's reach to new devices and databases. If a file name, record name, and device type were passed to an Open I/O handler, along with an I/O buffer, then it would be possible for 3rd parties to write generic Open I/O handlers to implement various data management and device management operations while maintaining a familiar, streamlined, and consistent programming interface.
This topic has triggered a number of ideas about extending RPG's reach to other database platforms, and UI devices.