As far as I know, WHATWG have deprecated some elements not widely used (like "isindex", "font"), but the documents using these elements will still be readable even if not exactly as the author intended. Moving to XHTML, on the other hand, would make billions of pages totally inaccessible.
There are people who are now dead who have pages on the internet. These pages will never be updated.
blink also doesn’t work anymore, and neither does marquee. Several of the frame attributes are broken as well. noscript doesn’t always work reliably depending on browser.
XHTML would have worked the same way – after a few years, you can deprecate the old parsers.