The main advantage to Flash is that you get a much more mature language (Actionscript 3) to work with over JS and you also get a whole host of great APIs that are easy to use. (From ByteArrays, to the Camera class, to different blend modes and filters, easy Bitmap manipulations, etc.). Now, JS is starting to implement some of these things which is good, but they still vary across browser, only work on the most modern browsers, etc.
So the advantages of Flash over JS (in my opinion) in short are: better language, better APIs, no tweaking for different platforms.
Anyway, if Shumway was fully featured, it would definitely be a big benefit. But if it doesn't work cross-browser and doesn't support all the features of Flash/Actionscript 3, then you're losing some of the main advantages Actionscript 3 gives you, and you're almost just as well off righting things in JS.
First of all, there are more things than typed arrays (vectors) in AS3, the main thing being a consistent runtime. Also the way you structure an AS3 project is different since the language supports packages/classes etc while in JS you (for better or worse) don't have these concepts so you end up with many different code styles. Even though browsers implement the some of the features, there are subtle differences in the implementations. This results, in real world scenarios, that this stuff are being abstracted away, layers and layers of abstractions, libraries for everything. I have my doubts that this SWF interpreter can be efficient.
There are too many layers in all doomed software, whether written in JS or AS3.
Subtle differences in implementations, you say? That is called the open web. Come on in, the water's fine. Pay no attention to the drowning plugins over in that cold corner...
Sure, but from what you said I'm guessing that they don't work in IE9, which is what I mean about cross-browser issues. Plus, TypedArrays are somewhat clumsy maybe?
It seems as if JS is the future, and that's not necessarily a bad thing, but it would be nice if JS had a few more features to make it easier to use. I personally wouldn't be opposed to starting clean (with something like Dart) but that's probably unrealistic.
Overall, the reduction of plugins is a good thing, who doesn't like less security holes, less software to update, etc. But it's also a bad thing if we lost features off stuff, lose functionality, loss cross-browser capabilities etc.
Also, can't believe I said "righting" instead of "writing" in my last post. That's quite cringe-inducing.
So the advantages of Flash over JS (in my opinion) in short are: better language, better APIs, no tweaking for different platforms.
Anyway, if Shumway was fully featured, it would definitely be a big benefit. But if it doesn't work cross-browser and doesn't support all the features of Flash/Actionscript 3, then you're losing some of the main advantages Actionscript 3 gives you, and you're almost just as well off righting things in JS.