I’m not an expert on accessibility, but I don’t understand this feature request. What is the difference, effectively, between alt and figcaption and why are you advocating that we diverge from CommonMark? What am I missing?
When I upload a photo, say of my cat, it is given a caption which is associated with the photo. I can then edit the caption to share context.
The caption is shown on mouseover (desktop) and at the bottom of the photo, when you select it to view it bigger:
