The fact that people didn't realize that a site used XSLT before the recent drama is meaningless. Even as a developer, I don't know how most of the sites I visit work under the hood. Unless I have a reason to go poking around, I would probably never know whether a site used react, solid, svelte, or jquery.
But it ultimately doesn't matter either way. A major selling point/part of the "contract" the web platform has with web developers is backwards compatibility. If you make a web site which only relies on web standards (i.e. not vendor specific features or 3rd party plugins), you can/could expect it to keep working forever. Browser makers choosing to break that "contract" is bad for the internet regardless of how popular XSLT is.
Oh, and as the linked article points out, the attack surface concerns are obviously bad faith. The polyfil means browser makers could choose to sandbox it in a way that would be no less robust than their existing JS runtime.