You should have been around before chrome books when any extension could do whatever it wanted without any permissions at all. Your understanding of history is missing some key pieces. Over time, Google has generally locked these APIs down not opened them up.

Browser JS definitely never had the ability to unmount storage volumes before Chromebooks existed.

Oh but your Java applet surely could.

And nothing would prevent an extension from loading an applet

Extensions could, and this action could be triggered from browser JS.