My (not super informed) understanding is that it may also depend on how popular the video is - the increase in encoding time is less worth it if there are few people seeing quality or bandwidth benefits (depending on how they tune it).
The client readiness is there. Already at 75% (relative to H264 100%, HEVC 15%).
It is thus clear which of the new codecs won client-side; it will only grow from here.
Definitely worth it.