> that sandboxing should be a first-class integral part of every harness,

Essentially, what you want is that the people who produce harnesses like OpenCode to also implement proper sandboxing and security. Why... why do you believe they'll do anything remotely satisfactory?

It's been empirically demonstrated, again and again, that at least 85% of software developers (and this is a very generous estimation) can not be trusted to bother about the security, so leaving the implementation of the security mechanisms and policies up to them simply doesn't work in practice. It just doesn't.