The debate over HTML5/Flash seems to consistantly ignore one thing: the reason why flash was so popular and powerful to begin with. Part of the reason, at least, was that it was such a pain to keep up with the different implementations of features, and when you were caught up, it turned out that you couldn't do the things you wanted to do. Flash allowed people to write truely browser-independent code. The ability to write code across platforms is something that made java popular as well.

Will that problem go away with HTML5? It's possible, but unlikely. Some companies/browsers dislike standards. Others think they are too cool. As long as there's a problem with compatibility and no clear winner, I think people will still use flash.