Nothing filled the gap left by Flash. It was easy, accessible, and drew so many young people to create.
Flash had security issues, but they could have been fixed. Macromedia and/or Adobe could (and should) have published an open source standard and realized their tooling would always be ahead of the game. They didn't see the incredible opportunity and dropped the ball.
Flash was so much simpler than the Javascript "standards" layer cake we have today. I'm not deriding the web -- it's the last free and open space we have left in an ever-eroding tech land grab. But Flash missed a chance to contribute to its defense.
If Flash had been in decent shape at the time smart phones arrived, Flash support might have been a required feature. Lots of people would have launched apps as Flash apps instead of iOS- or Android-specific apps, and we might have gotten much more open and cross-platform devices as a consequence.
I kind of blame Flash's lack of follow-through for the App Store hell we have today.
The web that could have been...