Not sure I agree. If you use some of the good transpiling languages that output JS, the syntax is far better than what AS3 had. In TypeScript/Flow, for example, you get "real" generics syntax while in AS3 you had the magic Vector<> stuff and that's it. And on top of that you have union types, dynamic interfaces, etc.
And thanks to JavaScript VM performance, running the same code is ahead of what the Flash VM could do.
AS3 was way ahead of its time when introduced, but browsers have moved past it since it stopped evolving.
Flex vs frameworks is more of a matter of preference and contextual requirements so I can't comment on that.
I do not disagree, but the comparisons are not fair,first though , I am not advocating of people building new stuff with Flash so I am not trying to convince you to use it in present day.
-comparing a new language TS with the old AS3 is not fair, when AS3 was around TS was not yet here, also AS3 is compiled and in theory games that are well coded in AS3 should be faster
-comparing new js frameworks and libraries that are not built-in to the platform with built in Flash/Flex feature is not fair, we should compare the built-in stuff in the browser vs built-in stuff in Flash/Flex , and if we add frameworks for html/js then there are frameworks that people used for Flash/Flex.
P.S. If I had a choice in what tools to use in a new SPA I would chose today TS and React(or something like react), if you have experience with React are there good components already built, I mean good that are not buggy(dropdowns going offscreen, or if you have more then one component things go crazy), sometime back when I checked there were only basic stuff and wrappers for jQuerry quality ones.
And thanks to JavaScript VM performance, running the same code is ahead of what the Flash VM could do.
AS3 was way ahead of its time when introduced, but browsers have moved past it since it stopped evolving.
Flex vs frameworks is more of a matter of preference and contextual requirements so I can't comment on that.