So my point still stands — Qt on WebAssembly is analogous to the plugins of yore like Flash and Java applets. WebAssembly is kind of like a new plugin container for native code.
No it’s not. With WebAssembly, it is a matter of trust in your browser vendor and its ability to properly sandbox WASM/JS content. With plugins, you basically gave up full control of your machine also to whichever plugin vendor. The number of trusted relationships is different - it’s fewer with builtin WebAssembly. That is a noteworthy distinction, I think.
I think, it could have been Java (or the JVM at least) in place of WebAssembly in browsers already a long time ago, were it a more open standard.
Java itself is a very open standard (OpenJDK, Java language standard, etc.). Openness of the standard wasn't the problem. In fact, it offered a much stronger uniformity of experience no matter what browser you used, unlike web technologies of the time.
The security issues with Java and Flash were the final nail in the coffin, but they had already been fading -- Java due to UX issues and startup lag, Flash due to it simply being a divergent technology from the web itself.