The Weimar Republic certainly had some issues right from its foundation, and there are some serious failure points that contributed to its demise, but claiminbg that it was doomed to end the way it did really seems like an attempt to deflect blame.

The Weimar Republic did not fall prey to an inevitable doom, it was overthrown by a fascist coup when the nazis managed to persudade/coerce non-fascist right wing and centre parties to support them.

The desire to create a kind of Ersatzkaiser in the person of the president certainly played a role in the rise of the nazis. Hindenburg had far reaching powers and was persudaded to wield them in the nazi's interest. Given that the nazis were not reluctant to actually break the constitution it's not entirely clear that having more checks and balances in place to prevent abuses of power by the president would ultimately have prevented Hitler's dictatoship, but there would probably not have been as clear a path, especially without an absolute majority in the Reichstag which the nazis failed to achieve again and again.

But there were problems long before the nazi's rise ever began: the military kept a prominent role in post-World War I Germany, starting with the fact that Hindenburg did become president, but also apparent in the establishment of the stab-in-the-back legend which shifted the blame for the lost war from the military to civilian politicians (and was later used to great effect by the nazis), and the leniency towards the Freikorps, even after attempted coups and assassinations of prominent politicians. The militant right was allowed to establish itself in the new state.

Yes, there were also militants on the left, and coup attempts like the Spartakus rising and the uprising of the Red Ruhr Army, but those were suppressed more vigorously, including the killing of prominent leaders like Karl Liebknecht and Rosa Luxemburg. Not only was there an imbalance from the beginning, but this also lead to rifts between the more moderate SPD and the more radical left which did not happen at the other end of the spectrum. This later allowed the nazis the find allies in the moderate right and also prevented the moderate and radical left from forming a united front against the fascist takeover.

So there were potential breaking points from the start, and growing economic problems did not help to alleviate the situation, but saying that the collapse of the Weimar Republic and the nazi dictatorship were already inevitable in 1918 seems like gross oversimplification at best