With Mozilla considering H.264 and Google refusing to drop it from Chrome, it seems unlikely that WebM will play a significant role in general-purpose streaming going forward. On a positive note, WebM did accomplish two useful functions.
First, it wasn’t until after Google released WebM that MPEG-LA announced that free internet distribution of H.264 video would be royalty-free, and I doubt that would have happened absent the viable alternative that WebM promised. Second, WebM quieted all those annoying Ogg Theora promoters who fervently (and incorrectly) believed that Ogg was the future of HTML5 video. Beyond those two achievements, however, WebM is truly forgotten, though not quite gone.