It has nothing to do with the specifics of the technology. As a consumer of online content, I don't care one bit if it is styled with XSLT or CSS (though as a developer my condolences are with the author, if they worked with XSLT).
However, what I do care about is that it _remains viewable and usable_. Imagine if Microsoft Word one day decided you couldn't open .doc or .rtf files from the early 2000's? The browser vendors have decided that the web is now an application delivery platform where developers must polyfill backwards compatibility, past documents be damned.
And just as the article drives the point home, it doesn't have to be this way. They could just provide the polyfill within the browser, negating any purported security issues with ancient XML libraries.