@DigWeeD, sorry I should perhaps expand a bit more.
JS plenty fast enough for display list management, but if you want to use the gpu for rendering you will run into issues with things like filters, blendmodes, potential texture switches and program changes and vector rendering.
Now if you used canvas without webgl then you can have pretty decent speed for bitmap/blitting but vector graphics is going to be slow by comparison.
Now not everyone uses those features fair enough but flash has traditionally tried to run the same api across all it's targets, so when you start putting caveats in like well you can't use this,that and the other then it starts to lose it's appeal and then you might as well just use any of the existing js engines out there. After all js/html rocks at things like ui and is good enough for simple games if you take care of your GC.
So then onto the issue of "use js instead", if you have large codebase it doesn't matter if you can port it to js or not if you are using any features that cannot simply be reproduced in js/html.
Now a great deal of code would probably be portable but I have to question weather it is in Adobes or Developers interest to have them invest in this undertaking only to find that a small percentage of projects are able to be compiled over without major issues.
Unity is able to target html/js (just about - their forums on the matter have been full of issues for ages) because unity runs on a different model of GPU only rendering.
Adobe was going to go this route and ditch the display list for asnext and go GPU only. We would have gotten a faster and more portable platform but they backpedalled out of it worried that it would break to much existing code bases.
If they had of pursued that avenue who knows where flash would be now... probably (maybe (maybe not)) everywhere.
They already are making use of LLVM I think in the compiler so probably easy to go from there to other targets... just the issue of incompatible apis.
So in summary, it would be great if we could cross compiles to a web output. I just don't see it working with the current feature set and I am not sure if a x% complete implementation will be enough.
Adobe animate CC exports to html in 2 formats I think including some code but still so much just doesn't work - I don't know if anyone is actually using those outputs for productions games/apps but you never know.
Even things like fonts, which were so easy to embed and use in flash, become a big problem.
To re-build the flash rendering engine in js in my opinion is just going to be big an ask without native speed. Trying to render crisp anti aliased text and vector graphics with filters and blendmodes is going to be slower than a dead snail super-glued to the floor.
*caveat: I could be talking out my backside - a lot of this is guessing and I could be wrong about absolutely everything