It is not as simple as that. Even if videos are prevented from being auto-playing, people resort to much much worse techniques. For example, GIFs - which are much bigger that the corresponding video, worse for battery life etc. To combat this, Chrome recently enabled autoplaying of videos if the video is muted on mobile browser. Hopefully this gets the number of people using GIFs down.

Or who knows, people might start animating images on canvas or something. Allowing auto play in some conditions seems to be lesser of two evils according to me..