that's not how statistics work. the fewer unpredictable drivers there are, the safer it gets.

if 100% of the drivers are human, every single interaction between two given cars is an at-risk interaction due to human error.

if 50% of the drivers are human and 50% are AI, then about 25% of the interactions between two random cars are between two AI drivers meaning only 75% of 2-car interactions are at-risk.

of course this is oversimplified, the vast majority of interactions on the road are not 2-car scenerios and the risk of interactions are more complicated than binary safe/not-safe situations. But the point still stands that the more AI drivers there are in comparison to more error prone humans, the safer it is for everyone.