Apple only has about 10% marketshare with macOS. There is no judge in the US that would rule against them in an anti-trust case over that. It's another story with iOS but that's not what we're talking about here.

Apple has no reason to turn the Mac into a total walled garden. They already have that in the iPad. Walling off macOS would mean getting rid of the Terminal and all of the other developer tools. For what? They would be cutting off their nose to spite their face.