The illusion that all designers are on Mac is probably 
  because HNers don't often meet the ordinary designers who 
  have a boring corporate job or are struggling freelancers 
  somewhere far away from San Francisco.

Out here in the midwest, every design freelancer I've known has had a Mac, without exception. Every small-biz graphic design studio or ad agency I've come into contact with has been a Mac shop.

3D artists, however, have been just as consistently Windows-based. I've never met a single one of them with a Mac.

That's why I said “most design studios” and not “all designers”.

I'm in and out of recording studios all the time. I haven't seen anyone running a Windows rig in probably 8 years. Everyone I know is running Pro Tools or Ableton on a Mac. And even back when I saw people using Windows in that role, they used external audio interfaces, just like they do now. Except with Thunderbolt, you get the expandability with effectively no latency.

I love Windows, but the recording industry lives on Macs.

I have friends that work in the audio industry… there's a nice mix of Macs and PCs but everyone uses external audio interfaces.