Catalytic Converter on Infinity QX56

I have been told by the dealer mechanic that I need to have both of them changed. I question the number and wonder if proper service could have prevented this going bad. My understanding is that there needs to be the correct ratio of air-to-gas and that if it is not it will cause havoc and also if the spark plugs are not properly maintained that it will also affect the results. My car has always gone through all the services and anytime it was the big one it was done through the dealer. Also I always put premium unleaded. There has been a history of computer problems in this car. Could that be a possibility? Also my lovely dealership never notified me of a recall on the AC electrical converter for the fan which may cause large amounts of moisture and water that may affect the performance of the car. Is any of this information such that I can get them to fix the car at their cost?