No one should freak out. John is leaving because that's part of the merger deal.
Sievert takes over. Sievert's been working with John for years. If Sievert wasn't good for T-Mobile John would have deep-sixed him a long-time ago.
No one from Sprint is taking over anything, although Masayoshi Son (who runs Softbank, which has close to 80% ownership in Sprint) has a spot on the new T-Mobile board.