I think it’s ridiculous to design a syntax that users would be explicitly warned against using.
the relative cost to the user of typing
!(image.png) is not worth the relative cost to the implementers to try to support and standardize a feature that we don’t actually want them to use to begin with.
My vote is to axe this extension. The premise of
Usually, nobody adds alt text to images in ordinary posts.
is flawed as it’s a failing of the author, not of the syntax.