The referenced comment does not even take into account that also the lossless compression is more efficient than PNG. This and the easy handling and efficient storing of (short) animations could in total combine the three major image formats: JPG, PNG and GIF.
Even when gifs are now mostly replaced by webm and HTML5 viideo tag, the unification of image formats for both orthogonal uses (natural images vs. technical or generated images) is a big advantage.
I'm still disappointed that you can't use (silent) videos in <img> tags or CSS properties where you can use images. Animated webp is so much less efficient than webm its not even funny. There are many places where you can already embed images (including GIF and animated WEBP) but video is not supported or has other restrictions.
But you can already do that. AVIF animations are regular AV1 videos. There is zero end user visible difference between what the above user is proposing and what AVIF does. Allowing webm in img tags would be identical.
Even when gifs are now mostly replaced by webm and HTML5 viideo tag, the unification of image formats for both orthogonal uses (natural images vs. technical or generated images) is a big advantage.