How about separating the physical network from the carriers?
Get someone like Crown Castle to combine all four existing separate networks into a single network, which the carriers pay for access to. Running the oily bits becomes Somebody Else's Problem, leaving the operators free to concentrate on services and customer service, and removes one of the reasons for your customers to churn.
Then give the physical network operator a landmass coverage target and uptime and performance SLAs, with financial penalties for missing either.
You'll probably have to have rules that stop one of the operators trying to become AT&T by buying all their competitors - no one operator can have more than 50% of the customers maybe?