Had the XHTML2 standard been adopted by many, browsers would have still had to support all the other non-X HTML documents, which would have never disappeared.
HTML5, with the exception of the new elements, just formalized the existing web parsing strategies for better cross compatibility.
As a web user, I don’t miss the days of XHTML sites randomly completely breaking because some tag wasn’t closed.
As a developer, I only miss XHTML2’s support for `href` on any element.
Ironically that was a very rare event in practice. For it to even possibly happen, three things had to come together which were still uncommon even at XHTML's height:
1. The page had to be written as XHTML
2. The page had to be served as XHTML
3. The page had to be parsed as XHTML
Usually at least two of those things weren't happening.
The vast majority of sites were "HTML 4.01" others were at best "XHTML 1.0 Transitional" (which in practice meant the same thing). Those using pure XHTML were relatively few. And of those who did, no major site served it as such because it would have locked out IE users, IIRC.