It was a `as_manager()` argument at some point but later removed: [github comment](https://github.com/django/django/pull/1328#pullrequestreviewcomment-5038269), [trac comment:22](https://code.djangoproject.com/ticket/20625#comment:22), [trac comment:23](https://code.djangoproject.com/ticket/20625#comment:23) and [trac comment:25](https://code.djangoproject.com/ticket/20625#comment:25).

Having both a custom `Manager` **and** a custom `QuerySet` is a bit of an edge case, especially now that we remove the need for a custom `Manager` for most cases, so I thought it was better to make the `as_manager()` signature as simple as possible.

Also, I was leaving room for a `as_manager_class()` method if the need arises.