Opening up private APIs is a commitment that you will support those. Which is no light decision.
Personally I think it is fine for any platform to have these.
But I do wish that the App Store rules around them were more permissive.
From Apples perspective: look at all the shit they are getting for dropping 64-bit support. This was known for a decade and they still are pictured as the bad guys who don’t support developers. A decade!
So you can imagine what happens when apps relying on private APIs suddenly break.
You know all the talk about Google or Facebook or Comcast both owning the platform and competing on the content layer with self-dealing is anti-competitive monopolistic behavior? Same goes for Microsoft Windows in the 1990s (actually litigated these very issues) and Apple today.
This distinction does not matter in the public sphere. When Apple releases an update — any update — that breaks a popular app, they are blamed for it. By putting their foot down, Apple is putting the onus back on developers to play by their rules.
This situation is essentially the normalization of deviance [1]. If you do not enforce a rule, that rule becomes invalid and unenforceable. Apple does not want to lose control of their platform. If they give away all of their private, low level APIs, then cross platform frameworks like Electron will render their platform a commodity.
And WebKit is a fundamental OSX library used for many purposes.
Why should apple apps like Safari be allowed to use private APIs if they're not allowed?
This is blatantly non-competitive.
Also, they sit on these APIs for YEARS with no alternative.