I agree with you in that navdy will not eliminate risk of attention loss. But at least it should reduce the attention loss. Sure, any kind of attention loss is not acceptable in theory. But if navdy is found to reduce attention only as much as say talking to a co-passenger, then may be it's not a bad trade-off. Like one of the other commenters said, we cannot ensure that drivers have 100% attention. Anything that at least reduces risk is still a good thing.