Core Competency Should be Challenged
A Core Competency should be difficult for competitors to imitate. But I would like to add that it should also be challenged from time to time by the competitors. Otherwise it will fail to deliver desired results. History says innovation is important for sustanability. High quality and revolutionary innovations are not possible without challenges. Competition brings-in challenges which drive innovations and changes.
Competition and challenges to CC's are two main drivers of innovation and change.