it's as if nothing was learned from the XHTML debacle
Then html5 came along, providing all kinds of shiny goodies and saying not to bother with the tags. In the end, a more rigid standard would have been nice.. (Though this is mostly about the skin deep part of the standards.)
and what new capabilities does this new proposal provide?
It failed because the smallest error by a client after the fact was like a server crash. Plus it would have created a mild barrier to entry when learning html at all.
xhtml was entirely opt-in, people opted into it, then served broken content. xhtml failed because that broke content (from people who, again, had specifically opted into serving xhtml) was an utterly terrible for everyone involved, as the user would get a big fuck off page devoid of any content, information, or means of redress, and there was no way for administrators or authors to get notified that their content was broken.
Meanwhile HTML would usually let you do the things you wanted to, and if you noticed something was broken you'd usually be able to hunt down a contact form and send a notice.
HTML5 is not what killed xhtml, xhtml is what did that, because it was a dreadful experience all around and had absolutely no redeeming quality.
Hell, the W3C was so into xml at the time there was an xhtml5 serialisation for html5. Technically it's still there (https://html.spec.whatwg.org/multipage/xhtml.html). That was of great use to the nobody whatsoever who was interested.