Re: It's UNIX
If you use something like MacPorts you can, indeed, replace a lot of components that Apple only updates with new releases. But much of the code, and all of the GUI, is Apple's own and it is frankly pretty shoddy. For the last three months I've been trying to confirm my Apple ID so that I can use FaceTime to chat with my nephew: no dice. How about replacing it? Again, no dice.
MacOS is for many people and many tasks a great OS, but it has definitely suffered from Apple's recent attempts to turn it into an extension of IOS.