WASM won't be able to get any real level of adoption if you have to specifically write all your code to target it, which I think is what would happen if the sandbox is too restrictive.
If you can trivially move existing code to run in a WASM VM you get the benefits of isolation and a convenient artifact to move around for deployment and scheduling.
I think the history of successful platforms are ones that still supported the old way of doing things, while enabling a new and better way. Of course that comes with lots of tradeoffs that are often hard to stomach.
I agree that "WASM as a generic target platform" is the best way for WASM to gain popularity and actually get used.
However, that would make it end up as "Java but different". That's not a bad thing, there's absolutely nothing wrong with Java as a technological concept!
If you can trivially move existing code to run in a WASM VM you get the benefits of isolation and a convenient artifact to move around for deployment and scheduling.
I think the history of successful platforms are ones that still supported the old way of doing things, while enabling a new and better way. Of course that comes with lots of tradeoffs that are often hard to stomach.