Maybe it's just because I'm used to the old way, but putting the TOC in the sidebar causes me to ignore it. But also, my eyes can't just go straight down -- they're now also jumping side-to-side.
I agree with simplifying the navigation links for what most people need. Overall, I think the old way is a better experience, however, because I use the TOC a lot.
You should install Wikiwand chrome extension and see the magic it does.
If you create a Wikipedia account, you can choose the mobile style as a default. That one is much less cluttered. You can also add custom CSS there so you don’t need that plugin.
I tried to figure out what the design process is at Wikipedia. I still don’t fully understand it, but it involves a lot of consensus building and making little changes. Looks like design by committee, where the committee is hundreds if not thousands of volunteers.
I found the reason for why there’s no max-width applied to the main body text on two discussion pages. A few people didn’t like it for reasons unrelated to legibility (‘I have a huge 21” screen and want to use all of it’—this was several years ago).
Just like in many other projects, the real design challenge is not about the graphic design, but about putting a user-centered design process in place.