I feel like most of your answer is invalidated by

* the actual existence of frames (although those are deprecated)

* iframes (which are not deprecated, so seemingly doing declarative inclusion of HTML in HTML was not what was wrong with frames)

* imports in CSS, which share some of the same problems / concerns as HTML imports

* the existence of JavaScript with its ability to change anything on the page, including the ability to issue HTTP requests and be written arbitrarily obfuscated ways.