Thanks for the link to that page, which contains a large image of a screenshot using San Francisco as the system font.
To me, it looks really great as a system font.
The reason is that it is legible. That legibility comes thru the junk on my glasses and the fact that my prescription is out of date so things are just slightly blurry. Yes, I need new glasses.
But that's a proxy for people who don't see as well. Apple's serving everybody. And not all of us can see so good.
There's an argument that this line of thinking will take Apple down the path of mediocrity-- lowest common denominator and all that. But hopefully it's only an option and they continue to use the "Best" font (of which I'm not the best judge) for the system.
And then make San Francisco an option as a system font in accessibility.