They won't be able to make all cars retroactively do this, and unless all cars are using this the system is essentially worthless.
You're telling me that if there are 2 self-driving cars on the road with 20 old non-network cars, there is no benefit for those 2 cars to coordinate?
What about the two smartcars coordinating their shared view of all of the "dumb" cars?
"Car 2, this is car 1. Dumbcar 249234 is next to me, in front of dumbtruck 2352 and has decreased speed from 72mph to 30mph and dropping."
"ACK Car 1, dumbtruck 2352 is directly in front of me. Reducing speed to increase space between myself and dumbtruck 2352, preparing to change to lane 3 when available"
*Obviously, they wouldn't talk like this, but could assign tags, coordinates & speeds, and this conversation could occur in milliseconds.
The idea is that the more sensors you have, the better the system works. Google and others are doing this with a single moving sensor platform now. A second one just makes both of them work better together, treating the non-connected cars as moving objects that must be avoided.
Looking at it another way, Waze is quite effective even with VERY low penetration rates. Imagine how much more effective Waze would be if every car made after 2018 was automatically plugged into it?