The primary uses that I see, day to day, for Flash are:
1. Video players (which should be done natively)
2. Ads (which is an awful use case)
3. Fancy, but broken, font replacement (less so lately)
4. Weird, unnecessary utility, like copying text to the clipboard.
I don't see any reason to reimplement Flash in Javascript when all of these use cases can be better done in native HTML/Javascript already (1-3), or just not done at all (4). It seems like a huge amount of engineering effort to maintain an old technology that even its creator is migrating away from.