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.
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...
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.