In reply to mark k..

@mark It needn't break back-compat. The feature plugin (Customize Widgets Plus) that implements Widget Posts maintains back-compat.

Also, there's nothing that says that “content” is reserved for the posts table (though I'm not sure what content means)—the posts table is an objects table. Nav menu items are in the posts table and widgets are conceptually very similar. In 4.7 new post types were created for custom CSS and customize changesets.

Lastly I'm sure that we can store widgets in posts without a degredation to performance if we employ caching.