It's a combination of a lot of things. In theory, Adobe Animate still exists today, but it's wildly inaccessible both because of cost and its proprietary nature. There's also complications based on how PWA get deployed to mobile, etc...

But ignoring all of that, here's a quick list of (some) categories of things I still miss from Flash:

- nested animations/clips: The recursive nature of Flash made making animations very fast. It also opened up some workflows with AS2 that were not particularly scalable, but that were excellent for fast prototyping. This is the biggest thing I miss when I open programs like Aseprite. I can't begin to describe how great it was for programming to be able to embed code inside of animation, and have the entire thing be a clip that could be embedded inside another program. Doing stuff like point-and-click games with that workflow was so fast and easy, and it was so easy to build up animations with a lot of disparate parts.

- a content-first workflow: related to the above, Flash let you switch between coding and content very quickly without worrying about stuff like imports. The fact that the content/animation and coding were so synchronized was a different way of thinking about architecture. There were problems associated with it when it came to scalability and organization, but again, just very fast to work with, very accessible, and lent itself to a certain way of thinking about prototypes that had some big advantages. When you had an idea you could be switching back and forth from code to animation constantly, you didn't have to alt-tab or export anything.

- AS2: maybe controversial, because AS3 was pretty popular and a lot of programmers were campaigning hard that it was the "correct" way to do things, but again, I want to stress the weird combination of content/animation/code was part of what made Flash special, and abstracting some of that out and having more "professional" content pipelines and different ways of doing events and organizing everything -- I think something was lost with that transition.

- Also just, in general, the animation tools were good. I don't know of an Open Source (or shared source) tool that has the same level of animation tools. Blender is doing some really compelling stuff that I'm very excited about, but Greasepencil is not a substitute for Flash, it's going in its own direction, and it's much less accessible. Aseprite is great (being able to tag frames and export data about them is handy), but it has some quirks and annoyances around the timeline controls. It's also just a lot more limited, you're basically forced to do frame-by-frame animation. There are some programs that I see animators online using that seem to be very powerful, but they're often either quite expensive or so restrictive and narrowly targeted that I'm not even willing to think about using them.

- And again, I can't stress enough: even just as an animation tool, even forgetting about games, adding scripting to animations is useful. You can have a movie clip that repeats until you hit a frame and throw an event. You can have a hand-animated scene with a procedurally animated background. And that's all really, really fast to do because the way the UX is set up you just have everything accessible at the same time.

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I don't think it would be impossible to replicate some of the things that made Flash great, and if someone really took the time to sit down and think about what made that workflow what it was, I don't think it would be impossible to make a Flash replacement considerably better than Flash. Flash had bad audio tools, its data formats were all closed down, it wasn't really extensible, it's drawing tools could have been better, and for most of its life it was terrible if you wanted to do anything with non-vector content. There are big improvements that an Open replacement could make.

It would be an uphill battle, because the ecosystem surrounding Flash didn't just exist because of Flash. There are a bunch of different factors that went into it. And there are some APIs on the web that are still annoying. Mobile support for web games is still annoying. Controller support is not great, rumble isn't really supported. There's stuff we could do better. But the core functionality that old-school Flash needed is mostly there, and we could have (functionally) a Flash replacement today. It also wouldn't need to be a web app, you could have a Flash replacement that targeted the web for compilation that itself ran as a native app, and that would be completely fine.

Heck, you could have one that didn't target the web. Obviously I'm thinking first about the web, but if there was a Flash-equivalent Open Source prototyping tool that targeted Linux/Windows, that would also be great, and as far as I know that kind of tool also doesn't exist today.

It could be done, it would just be a lot of work to build, and tricky to get right and to get polished. The really weird reality is that as far as I can tell, no one's ever tried to build a Flash successor that actually imitated the features I care about in Flash. There are plenty of game dev platforms, some very easy to use, but very few of them have great content creation tools baked in, very few are experimenting with completely novel approaches to architecture like Flash's movie clip system.