The thing is that all of those video standards use more or less the same basic math operations, just in a different way. This means that a WebM hardware accelerator would look very similar hardware wise to a h.264 one but would have different firmware.
In practice, I am sure that the hardware companies will make one hardware accelerator that can handle both h.264 and webm through different firmware programs. So you wont need much additional silicon or power to handle WebM. It might cost more, if the hardware companies decide to charge extra for WebM support, but that surcharge will not be much, because (i) they do not need to pay license fees, and (ii) there is a lot of competition in that field.
Also, Google controls YouTube. So H.264 <video> YouTube could see a sunset at some point, which would be a BFD.
You're dreaming if you think Apple could ignore WebM if it became the de facto web video standard. It all depends if WebM becomes that popular.
hristov's entire parent post was about why this sentence should end in "for now."