I think there's a fundamental issue here that the posted speed limit is often not the same thing as a reasonable maximum speed that's appropriate for conditions. Speed limits are set for all sorts of reasons that don't often match up with safety, and even then, they're usually set for normal conditions; when you have great visibility, dry pavement, and low traffic, it's often not unsafe to exceed the posted limit; just the same as when there's poor visibility, wet or icy pavement, or heavy traffic, following the posted limit is unsafe.

Having an alarm that distracts the driver when they're driving a safe and reasonable speed is an unnecessary distraction. And it won't help drivers maintain a safe and reasonable speed in unfavorable conditions, unless this also comes with real time updates for pavement conditions, visibility, construction, etc.

For cars that have trouble exceeding the speed limit, it can be dangerous --- sometimes there are scenarios where exceeding the speed limit is the safest course of action; but experience with the vehicle would help you anticipate its capabilities, and it is what it is. Usually there's pretty clear feedback that the vehicle isn't going to go much faster, so you will tend to avoid putting yourself in situations where you need lots of speed.

Rental trucks governors aren't necessarily as obvious, but that's also more of a statement that trucks should have a lower maximum limit regardless of conditions and that the rental company doesn't trust renters to be aware of safe speeds for trucks.