Defense-in-depth is always best practice in security. The more layers the attacker has to break and the harder each layer is, the better. All layers can and will be broken.

Apple has spent a long time hardening the JavaScriptCore web sandbox to run untrusted code. We’ve come a long way since JailbreakMe’s web-based jailbreak, but ultimately memory safety requires participation from all parts of the stack and JavaScriptCore and V8 are still both written in C++. You can trigger memory-safety vulnerabilities in the host VM using guest code.

wasmtime is supposedly a hardened WebAssembly runtime written in Rust, but it’s also a JIT, and I have no idea if anyone has put it through its paces security-wise yet. The idea is that WebAssembly can have JIT-like performance without JIT-like security concerns thanks to a simpler translation layer and minimal runtime.

I could see an argument for dropping some layers if the VM isolation become stronger