Hey,
even if wasm is great, it has some downsides. The biggest one is, that there is currently (as far as I'm aware) no java to wasm compiler that is mature enough to support all required java features. This probably also won't change in the near feature, until wasm gets a proper gc interface (e.g. see https://github.com/oracle/graal/issues/3391#issuecomment-833265906 as reference).
Another problem could be, that some features are best solved platform dependent. E.g. things like controller support, getting user input, local file management or how the UI looks in general. There are maybe ways to overcome this somehow, but I think it would be at least a lot of work.
I hope this answers your question!