Anyone have a good explanation for why someone hasn’t built a true replacement? e.g. a good, widely used flash-like tool that compiled down to a HTML5 bundle of assets?
The most famous is probably OpenFL, which uses the Haxe programming language. It does exactly that. You can even use Adobe Animate to author content for it.
The main missing pieces, however, are:
1) The abstraction inevitably leaks. HTML5 is just a fundamentally fiddly bundle of tech and there's always this or that little thing that can shake loose.
2) The tech burden is fundamentally higher with an open source project no matter how you try. With Flash you never needed to even touch a command line or download anything other than the Flash authoring tool.
3) The community is smaller. Fewer tutorials, fewer people able and willing to help you out. It's respectable, but back in the day Flash's community was MASSIVE. You'd post on the forum and get a detailed technical reply in MINUTES.
4) Resources are thinner. Without Macromedia or Adobe's pockets individual developers just have to bear the burden of dev/maintenance themselves.
5) Flash portals are dead. Now everything's about app stores for software and baked-out video for animation.
It's still a great project and I use it myself, but IMHO it's going to take someone to dump a bunch of money and time on top of these ongoing projects to recreate all the magic Flash had. There's only so much a small community of open source coders can be expected to pull off.