Flash was often misused - and you could argue it made it too easy for bad coders or malicious actors to load up a page with garbage. But as a game designer - especially for being able to code and work in the same environment as the artists producing assets - it was wayyyy ahead of its time. And it's true that even now, unless you use some other closed ecosystem like Unity, there is nothing even close to the workflow that Flash allowed for packaging a game that would work on the web, Android and iOS. And not just games - all kinds of interactive apps and simulations that could live outside a walled garden (which IMHO was why it was really killed).
Yeah, we've sort of got the tools now, ten years later, to be able to do what we were doing in the 2000s. Although it's still a lot messier. You now need to pick an audio lib and a drawing lib and a networking lib and hope they're all still supported in a few years, or be willing to maintain them yourself. But what really gets me is the way Adobe killed it in the end, by silently adding a self-destruct date to the plugin several versions in advance. I mean, people should still be allowed to somehow load my old games and apps if they want to. But now I can't even load them on my new machine. Literally hundreds of thousands of lines of hand-crafted code, three gaming platforms I built from scratch, several private business logistics apps that were still in daily use, and the majority of my work output from 2004-2014, was obliterated with very little warning.
Right now I'm still rewriting a critical AIR app that's still used by employees onsite all over the country, the last one I have in the wild. There's absolutely nothing wrong with the app other than that the language was killed, and it's basically impossible for me to push updates. Doing everything again is just a huge waste of time. The client can't understand why we're spending the money and time to completely rewrite something that will look and function exactly the same, only it'll be in TS instead of AS3.
The reason there's not an open source AS3 emulator out there that you can run in a web page is that Flash did so many different things, as related by this little article. It let you code sockets before there were websockets, and was still a friendly place for your artists to make their animations. It was like a swiss army knife with everything in one place, working pretty darn well.