It's a mixed bag. The ideal usecase for Flash vs React is very different. 10 years ago with AS3 I could put together a graphically rich game with decent sound or an intercative graphic very quickly, it would be performant and available as a small download -- much easier than doing similar today though libraries like phaser have certainly helped the situation a great deal. It just wouldn't make sense to try to make a sprite based platform game in React. On the other hand, complex forms, modular UIs and dynamically loading content -- areas where React (etc) excel were possible in Flash (espescially with adobe's Flex UI framework) but never looked any good or behaved in a satisfying way (my feeling is that pursuing this kind of thing as a kind of 'look at us we can do serious software' move caused adobe to take their eye off the ball wrt where they could provide real advantages over web-standards based solutions). Further, colaborating across teams of any size on AS3 based products was hard -- you'd have to pretty much roll your own CI/ deployment stuff (which I guess was the case with JS at the time too).
In summary: Horses for courses.
As a language I really enjoyed AS3; solid class system, strongly typed, good refactoring tools and crucially a decent and standard library.
I may be old or disconnected, but it seems like the like of the flash animator community won't be seen again. Where will we find another Homestar Runner or Weebl?
It was the right tool at just the right time. It was well able to deliver animations and interactive multimedia with the one tool and ecosystem. While the JS/HTML5 tools can manage animations and multimedia, it seems like the environment, libraries and tools are much more spread out... which perhaps discourages animators, while developers are used to working in such a way.
No matter the disadvantages of a medium or technology, there will be somebody making something amazing with it. Homestar is a work of creative genius. It's good in a way to be rid of Flash, but something is always lost.
In summary: Horses for courses.
As a language I really enjoyed AS3; solid class system, strongly typed, good refactoring tools and crucially a decent and standard library.