It could've been slowed down a lot if Google made Youtube in WebM by default by now. But they didn't, and that's probably because they don't think it's ready yet. Too bad. They might've missed their chance, or they will miss it if they don't do it very soon.
How was Google supposed to do that? Flash (still) doesn’t support WebM, so there wouldn’t be any fallback. Google surely isn’t arrogant enough to make YouTube videos not accessible to more than half of the browsers out there.
There was and still is no realistic way for Google to switch to WebM. Not exclusively. (Yes, they can also encode their videos in WebM but h.264 versions are always necessary.)