Nothing in the world is perfect. Breathing and eating are no silver bullets for staying alive because sometimes people choke on their food and they could also breath toxic fumes. Still it's a pretty solid choice to keep doing both.

My point is that due process is tangential to fairness or reasonableness.

This may be a misunderstanding between us. What is referred to by “due process” is the whole and entire ecosystem of laws and a judicial system with trials, representation, judges and predefined appropriate sentencing structures.

The whole and entire point of all of this, is fairness and reasonableness.

The fact that mistakes are sometimes made, even corruption sometimes, does not really change things. If corruption becomes common the system starts to fail and either reforms are made or the system decays into authoritarianism.