Personally I'm genuinely surprised/saddened that nobody stepped up to make an authoring tool. Neither a proprietary company like Adobe, nor an open-source initiative.

Because - the flash monopoly is basically "there for the taking". The market's demonstrated a huge amount of interest. There's a runtime that can do it (the modern web). All that's left is we just need someone to build an authoring tool, and they'll instantly win over that huge group of users who've been demonstrated to exist (not just legacy users, but tons of new folks with a similar mindset). Building an authoring tool of course is incredibly difficult, but - given the potential payoff, you'd really think someone like adobe would be all over that, if only out of fear of being disrupted.

I'm just really surprised nobody's done it. I expected it 5 years ago. Hell, I'm even surprised how sluggish flash emulators have been to show up - we're finally getting what I think is the "right one" in the form of Ruffle ( https://ruffle.rs/ ), but I'm surprised the older Shumway project didn't take off. Then again, maybe we simply had to wait, because I don't think anything prior to WebASM would have really been appropriate for the job.

I think the best case scenario would be some sort of "commercial open-source project", like how Krita structures themselves, taking up the mantle of being a flash-like authoring tool.

Perhaps the "single file" feature is no longer very high on the wish-list of the average user, especially since people now tend to share stuff online rather than via physical media like in the old days; this could explain why there is no widely used library to support this use-case.

The single file aspect isn't really a feature for the end-user, the end user just gets content they like. The single file aspect is more about the workflow for the content creator , content aggregators like flash portals, and the ecosystem that affords. Flash portals are all but dead now, but that's another story for another time.