I don't want to pull Kev into this discussion because this is a Sterling specific thread. But I'll say this on the matter. My opinion of Sterling is based on 6 seasons of data (including the current season) - 2 seasons he was good for his age, 2 seasons he was world class and 2 seasons he has been poor or average. Kevin has also been here for 6 seasons - 5 seasons he has been world class and one season he was mostly injured.
The people who were ready to sell Kevin because of 2 average games need to take a long hard look in the mirror. However, if Kevin is poor or average for 2 seasons straight while earning 300k pw, then believe me when I say that I'll start thinking about his place in the squad too. I don't worry too much about that day though, the best midfielder in the world is the best for a reason.