Thanks for that great perspective, I can absolutely see the benefit of the new design in a cramped display or at lower resolutions. OTOH, the comparison at 4k is strikingly different.
The old wikipedia design predates consumer access to 4K monitors though.
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 new design vs the old design.
https://imgur.com/a/rV1UXc4