I don't think the mouse chording is that essential to Plan 9. It's how rio and acme work, but those programs can be replaced without throwing away most of the OS concepts. I think you could absolutely create a Plan 9 UI that, for example, uses vi-like key bindings to jump around windows and text. Nobody's just done it yet and hardcore users are happy enough with what they have for now.
I don't think the mouse chording is that essential to Plan 9. It's how rio and acme work, but those programs can be replaced without throwing away most of the OS concepts. I think you could absolutely create a Plan 9 UI that, for example, uses vi-like key bindings to jump around windows and text. Nobody's just done it yet and hardcore users are happy enough with what they have for now.