Their online tech, bundled apps, and the Aqua GUI style don't need to be opened up for macOS* itself + the Kits + Finder to be open sourced.
If people could reliably and legally install it on any PC they want it could still cut into Windows' share a lot more than it currently can.
It's not hard to imagine that before long, enterprising people will release custom "distros" of it, say with an up-to-date OpenGL, or even a Wine/DirectX emulation layer baked in so we can just double-click on any .exe and have it run natively.
* As I'm assuming/hoping it's going to be called starting June 13. They could open source "OS X" while keeping the "macOS" brandname for themselves.
My point is that Apple has no interest in doing any of that.
> If people could reliably and legally install it on any PC they want it could still cut into Windows' share a lot more than it currently can.
But Apple would lose a huge amount of money on hardware sales, which is where they make their money. Apple even tried an approved clones program in the 90s, it was a miserable failure and one of the first things Jobs did on his return was kill it.
> It's not hard to imagine that before long, enterprising people will release custom "distros" of it
Which Apple really wouldn't want. One of the selling points of OS X is the lack of variation in both software and hardware.