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.