System Settings only uses
half of my 13" MacBook's screen width. That seems
entirely reasonable. There's
no shortage of pixels here...
And why would you have to mentally put the sidebar together with the left side of the screen? Do you complain about tabbed dialogs not having the tabs align with the top of your physical screen?
When you have more "tabs" than can fit horizontally (e.g. more than 5-7), the best solution is to turn them into a vertical list on the side where you can see and scroll more of them comfortably. This is better. (Whereas icon-based "table of contents" that disappears when you navigate and requires a "back button", as System Preferences was, is just terrible all around.)
You know all the way back in System 6, Control Panel used a left sidebar, and the overall window was a little more than half as wide as a Mac SE's screen. It's a pretty classic layout.
The new System Settings is totally clear on how to use, and totally reasonably sized. No unreasonable "mental effort" needed. Sure you can nitpick the precise organization of a few panels but for the many hundreds of settings, nothing will ever leave you perfectly happy.