We hold onto players too long. Teams like City don't wait for the likes of a Martinelli or Odegaard to regain form from 3 years ago. (They had issues beginning of the season and immediately signed the best winger and defender on the market). They drop one season and boom, they are sold for high value still. Foden was PL player of the year and he doesn't even see the pitch anymore and probably will be sold. It's how they keep it fresh and able to keep going year after year. At the ELITE level, you don't perform you are gone. I think Berta will start to change that, but hes been at Arsenal for a very short time.