There isn't always an actual need to change a design, but people feeling that something "looks outdated" if it's not following current aesthetic trends is a real thing.

Somehow the Wikipedia design doesn't seem to be suffering from that. (And that's probably a good thing.)

Good question. Counter-example: news sites. Very lean in '90s and early 2000s, but now still serves the same thing but with more tracking (so JS), autoplaying video for another newsitem (so additional data is consumed + battery life), and the essential content (what an average user wants) is still the same. Complicated visualization done wih JS+Canvas? Fine with that. News item? If it is a human, it would be obese already.

TLDR: News sites for reading news has certainly got heavier.