I disagree. Once in HTML5, the tags themselves could be ignored, folded, etc. Much like text banners can be collapsed via the AdBlock Plus Element Hiding Helper.

A good enough browser would have an option to not begin download of the movie until the are has been clicked, a la FlashBlock.

I see no reason to make blocking harder. If anything, it will be easier, as it will no longer be a flash object, but a video stream, much finer control, less of a vulnerability.

Speaking of vulnerabilities, any browser not allowing screening of images and videos is going to be exploited like in 1996. Blocking will be pretty much mandatory.

And if all else fails, network filters will work just fine.