Blu-ray allows for three different video codecs (MPEG2, VC-1, and AVC), there's no reason what ever UHD disc format is settled upon can't include both VP9 and HEVC, nor why streaming services can't pick between them. Which ever one is easier to use, with less cost, will win out in the end.

If you use Chrome or Firefox and visit YouTube you will eventually be getting VP9 video. So that's millions and millions of streams right there depending on it.

Pioneer PDP-5020FD, Marantz SR6011
Axiom M5HP, VP160HP, QS8
Sony PS4, surround backs
-Chris