how much is getting major attention from the decision makers for new releases?
Unlike other aspects of development, accessibility isn't binary (black or white - right or wrong). http://make.wordpress.org/accessibility/ isn't simply for discussion. It's intended to allow those with the right skills to prioritise and refine accessibility issues so that these can then be presented to the core team as specific recommendations rather than vague requests.
One tiny thing with a big impact would be to have the alt text field be mandatory
Over my dead body! Are you serious? It is considered best practice to use a null alt text for decorative images - not a populated one. And there's no way that any UI can automate an alt description - or even decide if an image should have one or not. This is way outside the remit of an UI. It can only be addressed by educating users.