Depending on what you're watching, you might never come across a video with good subtitles but rather Youtube's auto-generated subtitles.

Whisper can do a better job in a lot of cases, but not all... I wonder if they've had multiple generations of auto-captioning and not gone back and redone the ones that were done earlier.

This extension is really interesting to me because in the past I've tried (and failed) to make a similar one that adds a new .vtt to the list of available subtitles for the video. I sometimes struggle with auditory processing, especially in a noisy environment, and following along with subtitles helps me out immensely, so it's frustrating when the auto-generated subtitles are poor quality. I've bookmarked the extension to see if I can fork it for that purpose in the future.