By "usable" I mean having the bare essential features to be the primary operating system on a supplemental computer; to me that means having the features necessary to edit notes on the go. (For which I need a file system and a text editor, both of which are non-trivial but I think good overall goals to show that the OS's framework is complete)
Like currently AbleOS's skeleton is still being built, major parts of its infrastructure aren't done yet and are purely theoretical. It's better to have a complete vertical slice before worrying about supplemental features. I think "notetaking-OS" is a simple enough slice to show that the infrastructure is ready to be added upon. Given how much simpler PS/2 is compared to USB, I think effort that would be spend on USB and better spend elsewhere.
That's just my opinion though
