This is a great post to illustrate the utility of detailed alt-text when an image is the primary content. I think it’s less relevant for the more common use of images: as illustrative examples or supplements to surrounding text.
Take an image embedded in an article. Sighted users are capable of viewing a detailed image and instantly filtering for the information that’s relevant to the surrounding content. It’s harder to do this with a blurb of text, so this situation would call for more a more brief description.
The opposite extreme: the most detailed textual alternatives I work with are image transcripts (e.g. for comics). Flat text is a poor fit; semantic HTML works better. I’m hoping aria-details gains traction in the future and settling for short alt + long aria-describedby text in the meantime.