I tend to agree that the base model will have Supercharging as a paid option, much like it was an option for the old S60. Supercharging will be included in the Model 3+ (or whatever designation they select) pricing structure, much like the tech package was offered to Model S buyers. However, for those people who buy the base model and forgo the Supercharger option, Tesla should offer Supercharger access for 7,14,30 and 60-day periods. This could be purchased via Tesla's website on credit card, so Tesla won't have to worry about billing and calculating per-kWh (or clock time) rates. Perhaps $30 for 7 days, $50 for 14, $100 for 30, and $150 for 60 days, or whatever. (Still much cheaper than gasoline.)
In any event, the Model 3 will not be available until Superchargers cover 95% of the targeted area for the United States and Canada. (I am not up to speed on the rest of the planet.) With an anticipated range of approximately 200-225 miles, Tesla will have to fill in some of the longer gaps along the interstates in order for the Model 3 to be able to manage the distance without concerns for range or a 1-hour charging stop in order to reach 95% SOC. More stalls in urban locations combined with shorter gaps (80-120 miles) should permit any Tesla to travel comfortably.