To be clear: I work with a lot of people day-to-day whose full-time job is killing Flash. But before Flash existed, the web was documents, not applications. Flash showed the world how the web could show us movies, slick animations, web-casts, awesome interactive games, online-radio, pod-casts, YouTube and Netflix.
Under the hood, Flash isn't great. But it enabled millions of web-developers to express themselves more freely, it drove forward the web as a place where you could write awesome applications that work on every platform, and made billions of dollars for Adobe in the process. And people forget that before Flash, any interactive content required downloading a new ActiveX-control, and all embedded media was embedded Windows Media Player.
And it's perhaps also a surprise that despite how many security vulnerabilities have been found - and continue to be found - in Flash, Flash made the world safer by making the overwhelming majority of Active-X controls obsolete.
Maybe Flash lived too long. But there is no question the web is better because it existed at all.