I just can't follow him there to 50+. Ingrained habit certainly is part of that, but not all of it. The Steam UI is not all that great in a positive sense, even without comparing it to legacy. It is mouse capable, so mission accomplished, but its not elegant mouse capable, for reasons already pointed out.
The main problem of Toady - he's outstanding coder, but awful UI designer.
Both versions of UI is overcomplicated and non-intuitive for newbies.
But graphic UI is not only overcomplicated, it lacks visual usability.
Why there's no hotkeys shown on icons/pictograms? WHY?
Every level of menu must be hotkeyed with a key or sequence of keys (nesting level-dependent) to be pressed without mouse.
Like this:
Also, scrolling speed in stocks is awful and (WHY?) can't be modified in inits.
And strings are very height-consuming because of icons on left side.
With smaller icons and without line-dividers the quantity of strings in list can be doubled per screen.
Every item in stocks must be viewed not by magnifying glass, but right click (with changing "close-window" to Esc).
Centering on item with double-left-click.
OR viewed with double-left-click and centering with right-click closing the window.
And all these excessive strings marked red must be deleted.
This is just a little bit of improvement that can be easily done (by Toady) to make new UI much more usable.
Also, sorting dwarves in labors have to stay permanent.