You are thinking of the most trivial of apps. For example I have worked on a web-based image editor that is practically impossible to make accessible to the blind. Not with current screen reader technology.
Another example is Reddit, where a good chunk of their content is just text memes overlaid on images. For compliance, would you force people to describe their images with an alt text before submission?
The better solution would be to do what Facebook does and have builtin tools that will annotate a OCR/description with ML or crowdsourcing. In an ideal world these tools would be available to everyone with no extra work.
Don't settle for making the blind wade through a pile of HTML and call that accessible. It would be a travesty if it became law to use alt tags, because that is so far from the best we can do. Don't lock the world into dead technologies.
