Earlier quoted context omitted.
Look at it this way, this won't cause any harm and there's some chance that it will develop into something useful and successful. Right now there's no reason to conclude that this will turn into something that's no better than Java.
Has a single sandbox ever been written that has not had an escape vulnerability? I don't know of any. Sandboxes aren't safe, and everyone thinks of them as being a perfect prophylactic when they provably, demonstrably, and historically are not, in any way. the solution is not to create another sandbox to run software in.
One advantage in this new push to build another sandbox is that Rust is leading the charge and is the de facto language of choice when building a greenfield WASM project. Given its propensity for memory safety and WASI's emphasis on capability-oriented security, I think the WASI team has a good chance of building something with a lot of value. Nothing here is novel (except perhaps Rust's extreme prioritization of memory safety) but that doesn't mean it can't or won't be an improvement over prior attempts.
I wonder how many of the previous sandbox vulnerabilities were viable due to some quirky memory manipulation techniques? Quite a few I would imagine.