The rest will be converted in due time. These videos aren't available as HTML5 H.264, either, they require flash anyway.
Yes, I use Adobe Flash, not because I like it, but because Youtube still forces me to use it. (most sites I visit don't need flash for other things than displaying ads, which I block with Adblock, OC) Try for instance, play any music video from VEVO or other content provider... Or even simpler, try to play any content from (most) youtube "channels". Are they converted to WebM? I don't think so!