Keep in mind that there's a reason for Wikipedia's slightly cluttered interface: it's meant for editors as much as it is for readers—although I agree that paragraphs are too wide, a better strategy for line wrapping than letting lines use the entire width of the screen would be good.

You may like Wikipedia's "mobile" site: https://en.m.wikipedia.org/wiki/Launch_loop

I understand a bit, but I think that's no excuse for the clutter :)

The mobile site on desktop looks indeed great, quite close to what I built (just sans-serif, but that's a matter of taste).