It won't. In a couple of months everyone will have forgotten the old design ever existed and the next time they redesign it people will make extensions to bring back the new "old" design.

It won’t because this is a decent redesign. Love the contents on the left. Maybe a bit to much padding but it does aid readability. If anything I might increase the max width on the text a bit.

That's a drop in the ocean. Even power users might not be aware of browser extensions that do that. Look at the number of people who use adblockers: it's a minority, even though advertisement is a major annoyance to absolutely every single one internet user.

Most people won't bother to figure out how they can change it, if it occurs to them that it'd be possible at all.

Most of Wikipedia's users don't know anything about how to use the MediaWiki software (aside from searching and browsing articles) nor are they tech nerds. They won't change it even if they do prefer the old one.