Sigh. It's getting tiring.
High usability means a system is: easy to learn
and remember; efficient, visually pleasing and fun to use; and
quick to recover from errors.
Then MorphOS has high usability. What do you want to achieve with all this?
Are you missing OS3.5/3.9 features? That makes MOS inferior in your eyes?
MOS offers you to resize/move windows by borders, select screens/windows from lists, even with small pictures, use all existing icons and widely spread RGBA png icons in additions, stunning ways of navigation in window contents, either by holding down the middle button or pressing it twice, when a special navigational image appears, and a whole lot - which OS3.5/3.9 does not have and cannot provide. Despite that I haven't said OS3.5/3.9 is inferior.
The interface is NOT inferior/superior but DIFFERENT, and to say: pretty modern. And new features are added continuously.
Not to mention, we're discussing OS4, MOS and AROS - 3.5/3.9 is pretty off topic.