I would argue that none of the desktop browsers are having an effect in this area. The only thing that is really driving the adoption of html5 <video> is mobile devices and their poor or nonexistent support for flash.
Many sites that serve mobile devices html5 video still serve flash to html5 capable desktop browsers.
Mobile devices will support WebM; simply because desktops/laptops still rule and there's tremendous pressure to support desktop technologies for content-delivery that are popular.
And I would argue that the majority of traffic for websites like YouTube comes from desktops / laptops.
Many sites that serve mobile devices html5 video still serve flash to html5 capable desktop browsers.