[SeanLuke]

> Mac OS came from Mach

I believe you mean OS X, not MacOS.

Apple could switch OS X to entirely proprietary underpinnings and few customers would even notice. In fact, OpenStep ran just fine on top of NT. [And also on top of then-non-open Solaris].

https://en.wikipedia.org/wiki/OpenStep

[mrunkel]

OS X = MacOS.

Apple confusingly renamed OS X to MacOS a few years ago.

[quadrangle]

obnoxiously, it's actually macOS, not MacOS.