After having discussed the traditions regarding twelve successors, we can derive the following points: The number of Imams for this Ummah shall be twelve, in a continuous line. Secondly after the twelfth of these Imams, the world shall reach its end.
According to the first tradition:
"The Islamic religion will continue until the Hour has been established, or you have been ruled over the twelve Caliphs have passed among you."
This tradition has fixed the period of Islam's supremacy till the day of Qiyamat and also fixed the number of Imams for this Ummah as twelve.
According to the fifth tradition:
"The Islamic religion will continue until there are twelve (Imams) from the Quraysh. And when they all die the Earth will swallow its inhabitants."
This tradition proves that Islam will continue till the end of this world or till the reign of twelve Imams and after them this Earth shall be destroyed.
The eighth tradition limits the number of Imams to twelve:
"After me there shall be Caliphs equal in number to the companions of Musa (a.s.)."
This tradition shows that there will be no more than twelve Caliphs after the Holy Prophet (S). And the Messenger of Allah (S) has clarified that the number of Caliphs will be limited to twelve and after these Twelve, the Qiyamat will surely occur and the Earth shall be destroyed.
Other traditions clarify the above statements.
On the basis of this, we have to admit that in order to supervise the affairs of humanity, one of the Imams must have a long life which is an unusual phenomena. Thus the twelfth Caliph and the successor of the Holy Prophet (S), that is, Imam Mahdi (a.s.) was bestowed with a long life.