My initial take: I actually like this a lot. The body text is narrower which makes for easier reading. The sidebar with an updating highlight of current place in document is really nice too. I don't mind the "wasted space": Wikipedia is primarily about reading, and any well typeset book or web page should have generous margins so the line length is not too long.
Text width is important, but this is not a solution, zooming and changing browser window geometry is.
Also, zooming and changing browser window size is, while only a keystroke away, isn’t something I want to do just to make a site that should be optimized for reading text more legible. It should just default to that way.
In short, I’m sorry this doesn’t fit your flow. That’s frustrating. But I think this is a better design. Matthew Butterick’s Practical Typography seems to support this view. [1] (MB is a lawyer, typographer, and Racket programmer for those not familiar with his work.)
[1]: https://practicaltypography.com/page-margins.html#on-the-web