Comic Strips

That's not the only reason it can't last. From Umberto Eco's Ur-Fascisim:

For Ur-Fascism there is no struggle for life but, rather, life is lived for struggle. Thus pacifism is trafficking with the enemy. It is bad because life is permanent warfare. This, however, brings about an Armageddon complex. Since enemies have to be defeated, there must be a final battle, after which the movement will have control of the world. But such a “final solution” implies a further era of peace, a Golden Age, which contradicts the principle of permanent war. No fascist leader has ever succeeded in solving this predicament.

And also:

To people who feel deprived of a clear social identity, Ur-Fascism says that their only privilege is the most common one, to be born in the same country. This is the origin of nationalism. Besides, the only ones who can provide an identity to the nation are its enemies. Thus at the root of the Ur-Fascist psychology there is the obsession with a plot, possibly an international one. The followers must feel besieged. The easiest way to solve the plot is the appeal to xenophobia. But the plot must also come from the inside: Jews are usually the best target because they have the advantage of being at the same time inside and outside. In the U.S., a prominent instance of the plot obsession is to be found in Pat Robertson’s The New World Order, but, as we have recently seen, there are many others.

Since it can never be at peace, it constantly needs to create an enemy. The worst thing that can happen is defeating that enemy. It still has to have an enemy to focus on, or else people start to notice that the leaders have no idea how to solve actual problems.

So it needs to create a new enemy, but to do that, they have to carve one out of the existing population. This means the "acceptable" people shrink each cycle. It doesn't take long before the "enemies" make up too much of the population to be sustainable.

Another reason is the mechanics of cult of personalities. They rarely survive their leader. Mormon's and Scientology are often cited as examples, but they're the exception. There's thousands of little cults nobody has ever heard of that dissolve when the leader dies.

Hitler had maybe three to five decades of natural life left when he took the Chancellorship. That's the longest it could possibly have lasted. Trump doesn't have that. He has maybe one decade left, and that's pushing it.