> The future is in the cloud though, and OneNote has always
  > (for the ~3 years I've been using it) worked this way.

No, it has not always "worked this way" and it still doesn't across the board. I've been using OneNote for ~13 years (since the 2000 version). And in fact, the Windows versions continue to support offline notebooks despite Microsoft's attempts to shoehorn people onto the cloud.

A rather common OneNote usage scenario I see is a OneNote notebook on a company file share, or a shared Dropbox folder. OneNote supports multiple concurrent edits of a single notebook.

Not everyone wants everything in the cloud.