i don't think most people remember how bad Javascript was, how limited web API were, and how different browsers implemented totally different flavors of JS and DOM and standards were weak.
Flash pushed the web forward and demonstrated that it could be used to deliver real desktop like applications,videos and games, not just text and images.
Obviously it also came with a catch: the player was proprietary and full of bugs and security holes.
Ultimately, Web API progress also staled when Flash died. You still can't mesure canvas text properly in most browsers and API such as file system and WebSQL were dropped, which is a horrible mistake IMHO.