Is it an upward transfer? Just because he is going to the EPL, it is not guaranteed hell be on an upward trajectory. He is going to a team that will most likely be relegated. It is not guaranteed hell start, the majority of the games hell be there for. Why rock the boat when you have regular playing time and still have exposure to the NT? Right now, Berhalter has him as a lock CB, as an MLS defender!! The lure of the EPL will not make you a better player, especially at his age.
He's limited in how far he can develop here. No guarantees of anything there, but at least there is upside potential.