wasm is a low-level architecture - it gives you a linear heap to play with however you want. If you want immutable data structures and GC on top of that, you can certainly do that, in the same exact way we do it on other platforms.

The weird Java desktop GUIs were weird not because they weren't native code, but because the standard Java UI didn't use native UI APIs, nor did it even try to at least look native (as e.g. Qt does). But, just as there are native UI libraries for Java, there could be the same for wasm. It's just too early to tell, given how immature the whole wasm-outside-of-browser story is in general.