The term "HTML5" is still such a generic classification it's tough to say what it will look like in the future, and what it will encompass. Adobe already realized Flash is dying and this is why they released Adobe Edge, which basically is an HTML5 version of flash. The thing with HTML5 is it still can't come close to the dynamic animations capable in Flash among other things. I think once Adobe Edge or similar technology can do what Flash does but better, Flash will die soon thereafter. Until then, there is currently no tool to take it's place.
HTML5 is great for making snappy web applications, but currently it just can't do advanced animations and is pretty limited in regards to the creative realm. If Microsoft wants browsers to be plugin-free, perhaps they should start making Internet Explorer not such a pain in the ass to design and develop for.
//side note: I've been a web designer for about 7 years so I'm speaking from my own experience.