HarmLessSolutions: Assuming a consumption rate of 5km/kWh a 4kWh battery will provide 20km max on battery power alone. Short commuting may work but anything beyond that it becomes a hybrid ICE and the freqency of battery depletion isn't going to be great for life of battery.
Your consumption range estimate is almost identical to Prius 2012 PHEV when new in driver selected EV mode.
I believe your hypothetical on battery is incorrect in practice for Prius PHEV.
I'm not aware of any problems of that nature. Toyota hybrid HEV are well engineered to maintain economy and battery life. I don't think Toyota PHEV will be different. They will have left reserve and self-recharge ability to avoid the common cases of life reduction. Clicking on several trademe sales for 2012 PHEV Prius looks like excellent life. This thread is about the RUC change and probably not the right thread for extending discussion on other aspects.