This is so obviously false. Counterexample: Robusta Coffee.
Why do you say it is obvious? Monocultures were selected at a specific point in time for their properties, which may include robustness, but it doesn't change the fact that they are basically giant colonies of clones.
Real long-term robustness in nature comes from genetic diversity, which ensures future pathogens with a particular affinity will not be able to thrive through entire countries/continents.
That also applies to people right?
Do you have some cloning technology we are not aware of? A mono-culture of genetically identical human clones is sci-fi.