They were succeeded by Wicked People and Good People

After Allah mentioned the party of blessed ones -- the Prophets and those who followed them by maintainig the limits set by Allah and His commandments, fulfilling what Allah ordered and avoiding His prohibitions -- then He mentions,

(خَلَفَ مِنْ بَعْدِهِمْ خَلْفٌ) (there has succeeded them a posterity.) This means later generations.

Commentary

فَخَلَفَ (Khalf) means bad successors, while فَخَلَفَ (Khalaf) means good successors. (Mazhari) Mujahid (رح) has reported that when the Day of Judgment would be near and the pious people would all be dead and gone, people will lose interest in prayers and indulge in all sort of vices and evil deeds.