I think most of us here who aren't self-important "UX designers" (or branding consultants) would agree with you, but the decision makers responsible for most of the sites on the Web disagree/don't care what we'd like. They want their site to look identical on all platforms and browsers, and to have their "signature" design language, to heck with what users might expect. It's why you see stupid things like pixel-perfect clones of the iOS "switch" control brought to the Web.
So, anyway, if the `<dialog>` is ever to have a chance at adoption, instead of the "div soup and 1000 lines of JS and CSS modal" we've had everywhere since 2008 or so, it really should be blank slate for the "UX Designer" who fancies themself a real artist can vomit their personal brand of "elegant but bold, minimalist, flat design" onto the DOM.
If it's not completely skinnable, they'll just keep insisting on building div soup modals forever.
