Answering myself.
It may have reason "just not adopted yet", although it may as well be "not worth adopting", because of the following:
WEBP gives better overall quality, but image distortions are different on low quality encoder settings, when compared to classical JPEG:
- JPEG gives uniform distortions all over the picture, on hard edges, soft edges and soft gradients
- while WEBP handles soft gradients and hard edges better than JPEG, it gives more distortions on the soft-edges. Because of that, image looks deformed.
Example: moon in the following image: http://xooyoozoo.github.io/yolo-octo-bugfixes/#pont-de-quebec-at-night&jpg=s&webp=s
As a side note: WEBP is used for video thumbnails on YouTube, but given the source is video, WEBP is more acceptable in this scenario, than encoding thumbnails for JPEG images.