I agree and I know how backward-compatibility is important.
var tmpl = $("#template_element").val(); // textarea, script, etc.
var element = $(tmpl); // "\\s*<" will accept "\\n\\t\\s<tags>text</tags>"
If this change will break someone's code, that is already wrong code or vulnerable code.
I've never seen $("sometext<tag></tag>") snippets over 5 years.
I found many XSS on famous web-sites or web-applications caused by this issue.
All of them expected that "wrong css selector will return empty element",
so I finally think this is a jQuery's vulnerability.
"wrong css selector" is just a bug, but "create unexpected html tag" is a vulnerability.
Dangerous operation must be performed explicitly. It will be clear which operation should have priority.
I would like to also hear other people's opinions.