Let me rephrase that, client side xslt in browser isn't underpinning much of anything. I agree there are more uses in the enterprise world, although i think most of your examples are more XML not XSLT (people really shouldn't comflate the two. XML underpins half the world). I've never heard of anyone using xslt on a microsoft office docx file.

I'd also assume the java world is using xalan-j or saxon, not libxslt.