JVM had to be installed, and it was far from seamless the way OP describes it.

The advantage of wasm is that all the people running Chrome today already have it.

Plus the java installer would sneak in some junkware/toolbars in the process

Yes, thankfully no web browser vendor would ever do such a thing.

The big difference is that the "Browser VM" has well defined permissions to access resources on the host machine. Just like phone OSes do today. Bluetooth API, Files API, Location API, etc.

At which point you have an OS... except adding the word ‘webassembly’ somehow imbues it with properties OS’s don’t already because somehow web assembly compiles to assembly with extra security with no performance reduction.

Webassembly doesn't require you to install Oracle software, or lag your computer for 30 seconds while it starts up... And it doesn't require Swing or AWT. So, there are those minor improvements.

Swing is a very good improvement over HTML UI hacks.

Java doesn't require Swing or AWT either.

The JVM doesn't compete with C or C++.

JVM Bytecode is not a low level bytecode. It doesn't support value types or aribtrary pointer manipulation within a sandboxed memory area. It has a lot of java semantics baked in. Performance in most cases is better than javascript but it still is in the same order of magnitude because you have little control over the memory layout.

Webassembly solves a problem that existing low level bytecodes like LLVM IR do not intend to solve. LLVM IR is not stable across LLVM versions and usually is optimized for a specific architecture.

JVM bytecode is stable but high level. LLVM IR is unstable and low level. WebAssembly is stable and low level.

They all solve different problems.