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.
Yes, businesses that already use h.264 and flash will probably continue using them. But the bootstrapped websites and startups will opt for the free stuff at least initially, and of all those bootstrapped startups some will be successful enough to make some noise. And then some of the established companies will have cost cutting rounds and will look at those license fees and think about whether they really need to pay them when some of their brand new competitors aren't paying them. At this point the iPhone will start looking bad for not supporting the newest and coolest video startups that are supported on android and Jobs will eventually cave too and add WebM support to the iPhone.
Everything in the web thus far has gone towards the lowest cost and easiest alternative that is still effective, and this will not be different.