Oh hey, my plugin's on WP Tavern! Thanks Jeff!

I think you've definitely hit the nail on the head regarding not completely relying on cognitive services, but I hope at the very least this plugin helps people think more about using alt text in a way that's helpful to low vision users, and I'm glad that you were able to get some accurate descriptions.

Regarding adding text to existing images in the media library, that's definitely something I want to tackle in future versions. At first this will probably take the form of a button on individual images to receive alt text. Regarding bulk updating previous images, however, there are many challenges:

1. What does the interface look like? Does the processing happen in the background?
2. How do I process images without hitting request limits?
3. Updating alt text in the Media Library won't update images already added to posts/pages. How do I make this clear? Do I attempt to update content?

As you can imagine, these are tricky problems, and a bit too much for a 1.0, but I'm game to tackle them in a future version.

The very next version however, will be dedicated solely to fixing that "Confidence" typo on the settings screen :|.