> to make macOS a closed system like iOS

The reason why that won't happen (but more and more warning prompts all the time because of this modern world...) is because macOS is a self-hosted platform.

Apple _needs_ macOS as an unlocked OS at the very least as a development platform to develop their operating systems on. Not having the self-hosting requirements allows to tighten the security model a lot.

Notably, with a Unix terminal, a huge problem for sandboxing is how to determine permissions granularly, whereas a UI flow has significantly better answers to that.