WASM may remain stable, but it provides no APIs to the outside world AFAIK. The DOM keeps changing, and if the APIs are implementation-dependent, then WASM doesn't guarantee non-breaking at all.

Another advantage that Zeta has is that it will come with useful building blocks for dynamic languages. With WASM, you have to implement your own dynamic typing, GC, JIT, etc, and the current WASM implementation doesn't yet support JITting.