Integrating Opus and/or VP9 into WebM removes one of the selling points they used to promote the format. WebM wasn't supposed to be a generic container format holding different codecs. It was specifically tied to Vorbis/VP8 to ensure everyone advertising WebM support would be able to play the files.

If they decide to add these additional codecs then there will be some players that can play some WebM files but not others. If that was the direction they wanted to head then they could have just stuck with Matroska for the container format.