It's not supported on the previous main version of Safari, so everyone following the "last two major versions" of browser support rule can't use them.
Also, it's currently limited to only dialogs and popovers (and custom events, but in those cases you need js anyway).
It'll be more useful once it can control:
- details (open, close, toggle)
- video (play, pause, toggle play state, set seek point, mute, set volume)
- select (open/close widget, set/unset value(s))
- input (open/close widget, set/unset value(s))
- all elements (add/remove/toggle/set a class/attribute)
It's never been easier to create a great site that doesn't require javascript, but hardly anyone is.
People who disable JS are probably a very tiny minority and of those who consume ads, an even smaller one.