Flash was great, but abused to do bad things (like all good tech)
The killer was the iphone not being powerful enough/having enough ram to run the plugin, and adobe refusing to make concessions.
What it got right:
Design once, looks the same anywhere
reasonably powerful scripting language
Vectors as a first party drawing primitive
abstracted OS hooks
This was it's downfall, because it was for the time heavy to run. Combined with advertisers wanting rich flashy adverts, meant it became the bane of people's life.
There is still no replacement that is easy to author, and works pretty much anywhere. Sure there are loads of JS frameworks that sorta do one part of what flash did, but none of them have the rich editor that allowed you to have such creative freedom.
The closest thing to it now is unity.
I worked as a Nailgun operative in a Palette factory for a few dollars with people shooting nails at eachother for fun. I seen turtleshell.com one day and at the time I was like woah.
Flash was my route out of garbage and I miss it on behalf of younger people. I remember making a circle and keyframes. Attaching bits of code to frames to make it do what I want. Remember living at my moms house in the bath reading Flash Math Creativity or Colin Moock's books. I spent so so long understanding the concept of an Object or a Class or functions that call functions.
I feel that if I went back today, I'd be assaulted by package managers, dependencies, build systems, frameworks. No easy and beautiful way to draw a circle that weekend and animate it to an oval.
It is a real loss.
Stuff moves on and I am a developer, but my interest in motion (Penner! Keith Peters!) gave me a real solid feeling of what feels good with interactivity and motion and sometimes I open that ticket but it is rare. Everything is just the same now and I solve the same problems many of you do over and over again. It's boring. No creativity, no inspiration.
It seemed to really lower the barrier for creator to make some cool (and really funny stuff) so for that, I'm thankful.