A scathingly obvious and wider angle point that is rarely made, is that the vaguely referenced "flash" term actually encompasses the entire Flash Platform, and that includes the Flash IDE.
Animators and other artistic types who use the technology to create complex cartoons and animations depend almost entirely on the IDE for their work. It is just simply not possible without it. End of story. It's not just the technology at the language specification level that matters! It's also about the "supporting technologies" in this case IDEs (I say "supporting" but the Flash IDE is very much a technology integrated with the rest of the Flash Platform).
If someone builds an HTML5 IDE that has features that rival the Flash IDE, that's great, but I'm surprised the point is so rarely made. Artists are as much part of the debate as programmers, and it's unreasonable to say it is not their business.
So, to answer your question directly: HTML5 would be considered a totally reasonable alternative - or even replacement - to a multimedia framework such as Flash if and only if a proper IDE is built for it, thus elevating it to a multimedia framework also.
It is just not realistic to make an argument without taking into account IDEs. If the question is specifically "Can HTML5 in it's completed state, without an IDE, be an alternative to Flash for applications apart from manually animated complex animations?" then the answer would most likely be yes and we'd all go home.