I don't think that's actually rational, in the sense of minimizing loss of life. If you ground all these planes worldwide, what's going to happen? Either people will take older planes, which are in worse condition, or they'll take smaller planes / alternative routes, which are more dangerous, or they'll take cars, which are much more deadly.

Grounding the planes only guarantees saving lives if people stay at home instead, which isn't going to happen. If Boeing legitimately believes these planes are still no riskier than older ones (and it sounds like they do and that they have a reason for that belief), they're saving lives by keeping them in the air.

If my routine maintenance breaks the steering wheel linkage in my car, does the resultant crash mean that the manufacturer did not build enough safety into the vehicle to protect it from steering errors induced by negligent mechanics?