Even for videos Flash was the best solution for a long time, almost every video sharing website used Flash video players until HTML5 video got developed.
So ironically the designer no longer just asks 'is Flash available on this platform?' but instead 'which codecs are available on this browser and platform?'.
Can you give some examples of what's wrong with HTML5 video streaming and these implementation issues? There's a huge community of video engineers working to make streaming video better, and we'd love to hear feedback.