Can you provide a source that mentions this as a reason for the success of the revolution?

"A crucial turning point in the history of modern Iran, the coup [of Mosaddeq] had a stifling impact on Iranian civic-nationalist and democratic aspirations and derailed the constitutional evolution of the country . . . It adversely affected the Iranian political culture, lending greater plausibility to prevailing assumptions regarding an insidious British and now American influence. The coup would be ingrained in the collective memory of most politically discerning Iranians as an imperialistically induced defeat, a humiliating violation, and a stark reminder that Iranians were not in control of their own fortunes . . . Less than two months after the coup there were, according to some estimates, 13,000 political prisoners in Iran, consisting of supporters of Mosaddeq and of the Tudeh Party." - Fakhreddin Azimi, "The Quest for Democracy in Iran"

Dr. Azimi grew up amidst the Iranian Revolution, and was able to get out due to a scholarship program at Oxford. I was lucky to have had him as my professor at UConn.

According to his book and conversations, I'd hardly call the Shah's policies "not hard enough." It is true that his more forceful policies were carried out during the 1950s, notably by General Fazlollah Zahedi. However, it was the understanding among Iranians that their democracy was being subverted, and that the Shah was a Western puppet, that led to the Iranian Revolution. Contemporary parliamentarian Dr. Hosein Pirnia: "Even a genuine dictatorship . . . was better than a fraudulent democracy."

So, it was this desire for self-rule and national dignity that really fueled the Revolution. It is true that the Shah relaxed his crackdown on political dissidents in the 1970s, but this is only because there had been such consistent activism for the previous twenty years. Given his actions from 1953 to then, the revolution was really only a matter of time.

The revolutionaries themselves represented a vast coalition of wealthy Tehran elites, university students, and rural religious groups. They were united only by their hatred of the Shah and suspicion of foreign powers. Once the Shah was ousted, the only unifying force between them was that they were Iranian, and that they were Shi'a. Unfortunately, this left a power vacuum that Islamic extremists were able to fill, as seen in the brief period of Muslim Brotherhood rule in Egypt after the revolution of 2011.

The Shah never lost the loyalty of the military (they kissed his feet as he boarded the plane to leave the country) so he could have cracked down in that sense. The barrier was that the Carter administration was not a strong supporter of the Shah due to the lack of any democratic reforms being implemented under his rule. Any tough crackdown on the protesters probably would have drawn the ire of the United States, even if some in the intelligence community would have supported it. The Shah really had no choice but to let the revolution go forward. There was a great documentary made about the revolution by Peter Jennings during ABC news' 20th century retrospective that goes over a lot of this. He flew in on the plane with the Ayatollah, and so had a really interesting perspective. A lot of this also comes from a very interesting class on the revolution that I am currently taking, so I'd be happy to answer any other questions on the era you might have.

As to the historiography, I don't see the Shah always portrayed as a tyrant. Some historians, and moreso political scientists see him as an Ataturkesque figure that modernized and developed the economy at the expense of political freedoms.