Because of the train crash in the real world, all the kings and queens who stayed in the real world returned to Narnia (except Susan who betrayed Narnia). All the kings and queens returned to Narnia, which was the end of the old Narnia. They returned to Narnia forever (where they also saw Britain and their parents), which was the highest realm.

In other words, all the kings and queens are reunited, and then Peter will stay in Narnia as his supreme king forever.