Those suggestions would probably be better if everyone building a website had infinite man power. But in the real world adoption is going to suffer if you have to make a separate document for web, print, a11y, etc. Then someone will say "Why don't we have a unified markup language that we can use to produce each of these documents for us?" And then XSLT will be invented. And what a mess we've made.
But if people are already putting in extra effort to get these features working in their unified markup language, then it isn't different (from the perspective of effort) to write separate versions. In fact, it might be easier, because at that point you're working in a language custom designed for the task at hand, not trying to shoe horn a spoken-word UI into a visual UI language.
