But why? What’s bad about this?

What I said, literally.

But the web can support many more advanced use-cases than that.

Which can be done with something embeddable, and not by breaking a hypertext system.

So you can see that other people have different needs to yours, but you think those shouldn’t be considered? We’re arguing about the internet. It’s a pretty diverse space.

If those people don't consider mine, then I don't consider theirs. If I must consider theirs, they must consider mine.

Look, I’m not saying that the web is the most coherent platform to develop for or use, but it’s just where we’re at after decades of evolving needs needing to be met.

That says nothing. It's a market\evolution argument. Something changes tomorrow and that will be the result of evolution. Somebody uses a different system and that's it for them.

That said, embedded interactive content is absolutely not better than what we have now. For one, both Flash and Java Applets were mostly proprietary technologies, placing far too much trust in the corpos developing them.

And today's web browsers are as open as Microsoft's OOXML. De facto proprietary.

There were massive cross-platform compatibility problems,

For Flash? Are you sure? I don't remember such.

and neither were in any way designed for or even ready for a responsive web that displays well on different screen sizes.

Nothing was. Doesn't tell us anything.

Accessibility was a big problem as well, given an entirely different accessibility paradigm was necessary within vs. the HTML+CSS shell around the embedded content.

Yes, but applet's problems in that wouldn't spread to the HTML page embedding it. Unlike now.

Today, the web can do everything Flash + Java Applets could do and more, except in a way that’s not proprietary but based on shared standards, one that’s backwards-compatible, builds on top of foundational technologies like HTML rather than around, and can actually keep up with the plethora of different client devices we have today.

I've already said how it's similar to OOXML. Only MS documented their proprietary at the moment standard of their proprietary program and made it open, while Chromium is itself open, but somehow that doesn't make things better.

And speaking of security — sure, maybe web browsers were pretty insecure back then generally, but I don’t see how you can argue that a system requiring third-party browser plug-ins that have to be updated separately from the browser can ever be a better basis for security than just relying entirely on the (open-source!) JS engine of the browser for all interactivity.

That's similar to the Apple walled garden arguments. It's valuable in areas other than security because of separating power between some browser developer and some plugin's developer. And fighting monoculture is also good for security.

Also people still use plugins, still separately updated, which still get compromised.

Also plugins can be properly sandboxed.

The idea that any old website builder back in the day was more “ergonomic” while even approaching the result quality and capabilities of any no-code homepage builder solution you can use today is just laughable. Sorry, but I don’t really feel the burden of proof here. And I’m not even a fan of site builders, I would almost prefer building my own site, but I recognize that they’re the only (viable) solution for the majority of people just looking for a casual website.

Sorry, I still do feel that burden of proof. Because for a static site like in 2002 I'd just export a page from OpenOffice and edit some links, and then upload it.