The writer’s claim is groundless as he says that all the Imams since the victory of Abbasids were regarded as Mahdi and that their death was not taken for granted and that their return was held in a constant expectation and a fervent anticipation. Yes, there was one incident in which the seventh Imam, Musa Ibn Ja’far, died in prison where he was being held by the orders of Haroon Al-Rasheed. Some refused to acknowledge him dead. They consoled themselves by fancying Mahdism in him, which naturally entailed a waiting for his reappearance.
In their grief they imagined that he would soon reappear. It is likely that some mischievous elements might have injected such a pang into their minds. There were very few who went wrong; and the wrong itself was so evident that it could not deceive many more. The Shia believed in the demise of the Imam, from Ali to the eleventh one Hasan Askari. Why not take evidence from history.
Later those few who had believed in Imam Musa Ibn Ja’far’s reappearance realized their mistake and acknowledged his death. In the case of the sixth Imam Ja’far Al-Sadiq, he himself time and again declared that he was not Mahdi the awaited one. The qualities of Mahdi he had repeatedly disclosed.