So generally it is possible that in the near future SFML will support WebP
I didn't say that (and I'm not the one to decide that anyway).
But how I see it: Especially for the near future, there are more important points to be achieved in SFML (just look at the GitHub issues), porting to mobile systems is an important milestone for example. On the other hand, WebP is still a rarely used format, so its support will be of little advantage for the average SFML user. Keep in mind that it requires an additional library dependency.