Rationale for

I am working on a new DITA implementation for FrameMaker and stumble across a number of design issues, which make me wonder about the use cases that prompted the committee to choose this particular design.
I have not yet found a use case that would warrant the rich semantics on an alternate text for an image that is presented by <alt>.
I have yet to come across an editing and rendering system that would do something useful with such rich semantics inside <alt>.
AFAIK, HTML merely offers an @alt on an <img>.
Would it not have been wiser to have an @alt on <image> and push the richer semantics to a sibling of <image>, as part of a <fig> ?
I really want to understand the design rationale for this and hear some convincing use cases.
If such rationale and use cases are any less than convincing, I would want to propose a redesign for DITA 2.0.
Jang