I think that for the most part I like this proposal. I have three 'concerns'.. The first two are mostly about impact and messaging.
The first is just that as steve said earlier it'd be effectively building in a pattern that would create something we historically advise authors to explicitly not to write manually. That could use a solid explanation of why everyone is ok flipping on that (and confirmation that they are) or I think that could go badly.
Second is that as it stands, for those people who spent a lot of time and effort using sectioning and made good headings that didn't rely on the phantom 'document outline' that never shipped, the beautiful AT heading relationships, the conceptual 'outline' that they built will get less beautiful. For example, given something like:
<article>
<h1>Apple varieties</h1>
<p>The apple is the pomaceous fruit of the apple tree...</p>