> I think XHTML failed because it didn't give web devs any new capabilities, so most didn't feel the need to learn it and do the extra work of getting their tags correct.

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.