XHTML 2.0 had a bunch of good ideas and a lot of them got "backported" into HTML 5 over the years.

XHTML 2.0 didn't even really discard backwards-compatibility that much: it had its compatibility story baked in with XML Namespaces. You could embed XHTML 1.0 in an XHTML 2.0 document just as you can still embed SVG or MathML in HTML 5. XForms was expected to take a few more years and people were expecting to still embed XHTML 1.0 forms for a while into XHTML 2.0's life.

At least from my outside observer perspective, the formation of WHATWG was more a proxy war between the view of the web as a document platform versus the view of the web as an app platform. XHTML 2.0 wanted a stronger document-oriented web.

(Also, XForms had some good ideas, too. Some of what people want in "forms helpers" when they are asking for something like HTMX to standardized in browsers were a part of XForms such as JS-less fetch/XHR with in-place refresh for form submits. Some of what HTML 5 slowly added in terms of INPUT tag validation are also sort of "backports" from XForms, albeit with no dependency on XSD.)