I have some sympathy, but not a lot, because I don't feel like this is a problem with the web.
Honestly, just based on what I used Flash for, the big thing I miss has nothing to do with Actionscript or any of its code or distribution. Distributing SWF files was no easier than it is today to distribute an HTML directory in a zip file, in either case the only reliable way to share your stuff was to link it on a website. And obviously, the web works for that.
So what I miss is not the code or the platform, what I miss is the animation pipeline. I feel like Flash was a relatively unique animation tool and content creation tool and the web hasn't really tried to replicate that, in part because it really has nothing to do with the web's capabilities.
What we're missing is probably not some kind of giant API effort on the web that would make Flash animations possible, what we're missing is a standalone program that generates animations for the web in a format where they can be manipulated with code. As a pure prototyping tool for game development, I would already rather be working in a program like Construct instead of Flash. I just don't want to use a separate program to generate sprite sheets, I want them integrated really nicely into the timeline and the coding environment.
But I don't think that has anything to do with the web at all. The web definitely isn't perfect, but with WASM and WebGL we have more graphical/processing power than Flash ever offered, and aside from some problematic areas like web audio, controller input, and other game-focused APIs that browsers have been slacking on -- what we're really missing is a development environment. Flash's development environment wasn't self-hosted in an SWF. You don't need a development environment for the web that is self-hosted in a web page. You just need a thing that spits out WASM. It's not the web's fault that nobody has made that tool.
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If all you're worried about is programming and Javascript, then Construct 2/3 already has more accessible programming environments than Flash ever had at any point in its lifecycle.
Construct isn't as powerful as Flash was. But setting up a quick prototype game in Construct is fast, and it has been fast for a long time, and that's the important part. The important part of Flash was not how powerful it was, it was purely about the accessibility. And Construct 2 is an extremely easy environment to learn to code in. Even as a seasoned programmer I liked Construct 2's drag-and-drop visual interface; it made it easy to visualize the overall shape of the code.
But again, Construct lacks that animation pipeline where you can draw, animate, program, and export all in one kind of unified environment that allowed you to jump around a lot to different tasks.
And of course, Construct also isn't Open Source so you don't know what the company is going to do in the future. That's a real problem, that's why I don't use Construct at all, even as I'm singing its praises. I will not use a proprietary development environment. But it at least outputs to an Open format, and the people who miss Flash are probably not going to be losing sleep over using another proprietary engine anyway.