Pedro_Fernandez A good summary, though I would correct one thing. Flash’s retirement in 2020 was announced in 2017, though the writing for that had been on the wall for years, and if Adobe had not made the announcement then probably browser companies were about to pull the plug.
Stage3D was the last major innovation in Flash, in 2011. It’s striking how little Adobe did to support it. They perhaps thought companies would port their 3D engines, like UnrealEngine, Unity, to run on Flash. But the impediment to that was ActionScript. Plus Unity and UnrealEngine are direct competitors.
Without them, and without Adobe doing anything, it was left to third parties, none of which did especially well. I have not been following them closely so don’t know the reasons, but official backing and support from Adobe could have helped.
And then there’s ActionScript. It is not at all bad for 2D, but it’s poorly suited to the sort of math-heavy programming needed for 3D and 3D rendering. It has hardly improved since AVM2 introduced AS 3, while Javascript, Unity and other alternatives have improved significantly.