You appear to believe that teaching ability is an externally originating quality, such that when the blessing of the gods of teaching falls on a professor, his endeavors succeed. I am afraid this is (probably) not true. The difference between effective and ineffective teaching comes down to the choices made in methodology (assuming equal knowledge on the subject between compared teachers) and a large majority of those are discrete, tangible choices. Like not using a page of text on the handout when a single picture would do.
I'd like to challenge your claim. While you could obviously achieve a lot despite being restricted to a chalk and board, inability and unwillingness to use more effective tools at his disposal is precisely the feature of a bad teacher. It is not the only feature, but it is one of them.