Clarification of SC 1.2.3

Hi Everyone--
We were discussing SC 1.2.3 (Audio Description or Media Alternative) and were puzzled by the A and AA requirements. I wonder if anyone has had a similar reaction.
In terms of accessibility, wouldn't it be better to require a transcript than an audio description? As I understand it, a transcript would provide a more complete rendering of a video since it would include both audio and visual content, whereas an audio description only includes visual content not contained in the audio track. A transcript would also be more accessible to persons using a Braille reader.

Was the decision to make audio descriptions a Level AA requirement based on how few players could provide them, i.e., the assumption that providing a transcript was easier, therefore more reasonable to meet Level A? Or was a synchronized presentation using audio descriptions considered to be more comparable to a sighted experience than an asynchronous transcript?
It seems to me that a transcript would be preferable to audio descriptions, therefore optional for Level A but required for Level AA, but perhaps I'm missing something.
Thanks!
Mike