YouTube has every right to create a proprietary YouTube client (which they do on mobile devices) and prevent browsers from accessing it. Don't serve files to my browser if you don't want me to use them.
Creators, for better or worse, are putting themselves at the mercy of YouTube, as serfs to feudal lords. I'm sympathetic, and happy to pay (and do) for your goods directly, but don't complain to me if your lord mismanages your affairs.
Even TV users had VCRs to save content "offline".