I think relying on system fonts makes perfect sense. I have been using a plugin to remove Open Sans for some time. So I really want to like this proposal.

But I'm confused by what it actually involves. In its current form, it doesn't revert to the user's choice of system font. That, surely, would just means specifying the generic sans-serif font. Instead, this proposal currently specifies a long list of fonts first, before resorting to sans-serif.

I don't see the point of that at all. If a user has chosen his or her system font, then that should be used, and that's what sans-serif does.

What this proposal does, by contrast, is force upon the user a font for the admin just because it happens to be installed on the particular device that the user happens to be using at the time.

So it seems that I will now either have to install another plugin to undo the new stylesheet, or else I will have to uninstall particular fonts from my devices just to make sure that they won't be used on the WP admin pages. Bizarre!