Tl;dr - please ditch the margins and bring the auto-scaling, because the proposal makes things worse on a modern desktop.
MediaWiki has a very archaic UX based on hardcoded numbers of pixels, small font, clutter and laughably small thumbnails with the 220px width coming from an era of 15", 800x600 monitors in the year of 2004. It is high time to improve.
Unfortunately, this very proposal goes in a very '''bad direction''' and makes life on a modern desktop only worse. Instead of improving the paradigm and giving a website adaptive to the users' devices, it is trying to recreate and already old tablet look of 2010 - with hardcoded pixels, unadequate for modern desktops. It is a lazy design, copying the worst errors of today websites.
Firstly, it ignores the nature of the website and the latest design trends. The fad of tablet-friendly, wide-margin design has been very noticable in the "content consumption" pages, especially when the high hopes were placed with a new form factor (tablet) and cost-cutting with the "mobile-first" design. But it never made it right.
It has never landed so well in the "workspace", where every bit of space is being apreciated; and nowadays you can see some calls for a retreat to different paradigms in design. Moreover, we can afford to not follow the mobile-centered design, and provide the UX adaptive to the users' device.
Secondly, it is not bold enough. The proposal disappoints me personally, because it keeps the MediaWiki archaic where it is really, REALLY long in tooth (non-responsive design based on fixed pixel numbers, UX clutter, too small thumbnails on modern desktop), while it is blindly following a yesteryear's trend, adding superficial hurdles to read and write the content because of IMVHO misguided pursuit for legibility. It is like taking the worst from the two worlds.
The most glaring problem is the usage of fixed px and wide margins instead of auto-fitting on a computer screen. This issue will be only growing with the growing pixel width and physical sizes of the desktop screens. Even already on a WQGA or 4K monitor the proposed page '''looks ugly''' with nonsensical wide margins. And it will get only worse.
The pursuit is IMHO misguided, as it is focused on "limiting the text width", rather than on the overall experience (especially font SIZE, spacing) and the general UX of a person using a modern, large desktop screen.
The proposal quotes some random text of an IBM researcher. It is not even a scientific paper, it does not seem to be even reviewed or reproduced; we don't know which year it is from and what actual environment does it represent. Are these thoughts based on a 14" VGA screen, 14" 1368x768 laptop, or a 28" WQGA setup? Was font scaled as well? What was the font, actually? We don't know.
I was honestly trying to find a source of these margins and "up to 75 characters in line" space, but all I got was an echo chamber with no contemporary, real research to back it. But I failed, I encourage everyone to provide me some papers.
What I got was the insight that actually:
* it's the size, font, spacing which really matter
* long lines seem to be pretty fine on monitors
* some claims are rooted in the old guidelines from the times of printing press and paper - not really relevant for e.g. contemporary use cases, sharpness and e.g. distance from the screen.
As you can see, the 18pt font was recommended for Wikipedia. No margins or artificial limits. Just size.
Any contemporary research on actual, contemporary desktops (Full HD, WQGA and 4K screens of 24"-29" size) would be appreciated.
On top of that, please test on some modern desktop: 27", 28" WQGA and 4K monitors, as well as on modern 14" laptops with the same WQGA or 4K. These are the standards in the beginning of the decade; not complying with them makes any proposal irrelevant.
When you will have tested, you will know that the proposed page is actually uglier. :/
Thus, as it has been mentioned before - we desktop users have our high pixel density, large screens for a reason. And we will be having them more and more often, because these are current IT trends. Instead of imposing silly margins, please work on proper auto-scaling and an adaptive UX design.