You do realize that Actionscript 3.0 is the same ECMAScript that Javascript is right? Actionscript 3.0 was based on Ecmascript 4th edition and embraced a bunch of OOP JAVA(TM)-like of sloppy coding that Javascript coders complain that Javascript doesn't have. This stuff got added in 6th editon (aka 2015 edition) which means that one could completely convert an AS3.0 Flash project into a SVG+Javascript project+WebAudio or Canvas+Javascript+Webaudio and you'd likely not be missing anything.
I'm sorry but your view on things is quite simplistic. I'd even risk saying that you're basing your statements on stuff you might have read on some wiki / random article without actually having any / much hands down experience with either. The reason I'm saying this is because anyone who has worked with the Flash technology ( Flash / Flex / Air - and by "worked" I don't mean people who are "expert banner creators" or the ones who managed to hack together some simple contact form / tetris / image gallery / video / audio player from N tutorials in M weeks / months ) would know that just because both languages are based on the ECMAScript standard, they are still quite different beasts (
not "the same" )... Had you said AS 2.0, then perhaps we could have agreed on more points... prototypical inheritance, lack of type safety, etc.
The programming language is just one aspect of the technology, saying that you could easily convert AS 3.0 projects to JS / HTML(5) / CSS only proves that your understanding of the technology and / or experience with it is quite limited.... either you ignore or you genuinely don't know how important Flash Player is in this entire picture. Flash's "magic" does not come from ActionScript, it comes from the Flash Player, from what it can do. ActionScript is just a means of communication with the player, they could have easily gone with JS, C, Java, C#, Python or whatever other language instead, had they felt like it...
That being said, sure, there are things that one could convert / replicate... this is only logical since Flash has been / is getting replaced by HTML(5) / JS / CSS content in quite a few areas. The thing is, that this "easy conversion" usually only works for quite basic things, the moment you try converting anything more beefy ( a proper web app / RIA / enterprise app / game, etc. ), then you'll see how fast you run into walls that you might not even be able to break... JS / CSS / HTML(5) is not yet ready to fully replace Flash. Even if it can / could ( technically speaking ), it's still a pain to work with when it comes to atypical / large / enterprise level apps / games...
... on the bright side, tools / libraries / frameworks / other languages are slowly closing the gap ( to name a few: npm, gulp, bower, typescript, angular, bootstrap ), but there's still quite a long way to go until things reach the needed level of maturity, stability, reliability, maintainability, etc. so that the industry as whole can really step away from Flash for good ( perhaps Flash - the technology - will never die, instead it will continue to live on under a different umbrella... with the help of Adobe Air; running on your desktops, phones, tablets, consoles, etc. without many of you ever knowing ).