"XSLT is currently the only way to make feeds into something that can still be viewed."
You could use content negotiation just fine. I just hit my personal rss.xml file, and the browser sent this as the Accept header:
text/html,application/xhtml+xml,application/xml;
q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8
except it has no newline, which I added for HN.You can easily ship out an HTML rendering of an RSS file based on this. You can have your server render an XSLT if you must. You can have your server send out some XSLT implemented in JS that will come along at some point.
To a first approximation, nobody cares enough to use content negotiation any more than anyone cares about providing XML stylesheets. The tech isn't the problem, the not caring is... and the not caring isn't actually that big a problem either. It's been that way for a long time and we aren't actually all that bothered about it. It's just a "wouldn't it be nice" that comes up on those rare occasions like this when it's the topic of conversation and doesn't cross anyone's mind otherwise.