Level 5 isn't the only safe level. Level 4 is safe too - e.g. a car that is fully capable of driving itself without human monitoring in slow stop and go traffic on a highway.
Levels 2 and 3 are the danger zone (and it worries me that car systems have gone ahead from level 1 to level 2, as having the human steer ensures driver attentiveness which is harder to maintain when the car does lane centering for you).
Level 5 isn't the only safe level. Level 4 is safe too
I agree that, by definition, this is necessarily true.
The catch I see is that the same definition is predicated on the vehicle being able to safely end the journey before entering any unsupported situation, without requiring any driver interaction. I'm not aware that we have any known strategy for solving that problem in the general case that would not achieve level 5 anyway.
I acknowledge that in specific situations like geofencing, where a vehicle does effectively operate at level 5 but only under predetermined conditions, that would be level 4 according to the scale. However, it's the ability to operate fully autonomously, albeit within those boundaries, that makes the vehicle safe in this scenario.
So, what happens if external conditions (for example, directions by a police officer, or some sort of road accident or severe weather) mean that the vehicle cannot safely remain within the area where it can operate autonomously? Unlike a vehicle with a human driver, it cannot adapt and safely leave that area either.
In short, unless perhaps we're also going to have a new set of rules and possibly some separated infrastructure for use with level 4 vehicles, I'm not sure they can ever fully match the safety of a human driver without necessarily reaching level 5.
Levels 2 and 3 are the danger zone (and it worries me that car systems have gone ahead from level 1 to level 2, as having the human steer ensures driver attentiveness which is harder to maintain when the car does lane centering for you).