The new design vs the old design.
The reason the text wasn't wider is because long lines of text are hard to read. It's why newspapers typically arrange their text into columns, rather than having one article being 2-3 lines long stretching across the entire page.
The problem, if there is one, is information hierarchy and deciding whether/how/where to disclose a given information at a given level in that hierarchy and how it’s disclosed. It’s very situationally dependent and probably impossible to please everyone. Optimizing it for the best outcomes for most users is what most good designs do, and they very seldom do the things a lot of vocal HN complaints want (shove everything on screen close together with small text, ie optimize for desktop power users; or make everything infinitely configurable, ie optimize for… idk even what the optimization serves, the same users complain about the web being overly complicated).
(It's perhaps worse in that way, since it's the tooling generating the HTML, of course, so this is just going to propagate…)
I can't remember going purposely back to the ToC myself, however I often enough scroll over pages to find a section again and there it helps to have the structure visible.
However the nice thing: In your preferences you can pick the style you like. So if you want the old one pick it.
You can also leave feedback with them that "random page" is important (I like that one too!) so they find a better place ...
Now instead of lazily clicking from one random article to the next (or to a different language, or to "current events", or the "main page" of headline articles), one has to move the mouse twice and make two clicks to get to it through the hamburger menu, or use the URL bar. I can't even remember the last time I went back to the ToC when reading a Wikipedia article and this change is utterly incomprehensible to me.