In this case, I think there’s an important difference when you factor in management.
More diverse systems are sometimes more stable than less diverse systems, but it matters how the system is supported. If it’s self-sustaining, it’s legitimately strong, but if it’s artificially supported by humans, it’s not. Humans, and our political, financial, social, and religious systems, become the weakest link. Instead of being built on a foundation, they’re built atop something far less stable.
Generally speaking, weak monocultures are those that rely on near constant human management, and threaten systemic harm via bottom-up trophic cascade because of their failure to survive: Agricultural crops and trees planted for forestry purposes. These monocultures are vulnerable to attack by a diverse horde of higher level trophic occupants and the pathogens they carry. The endangered species act basically incorporates listed species into this model of artificial human support. Works well with things like DDT, deforestation, pollution, hunting; things humans do directly. Not so much with managing other species to support target species. But back to monocultures:
Organisms that have evolved to form monocultures are typically successful and resilient. I can name the following highly successful invasive aquatic plants off the top of my head:
Typha angustifolia, and T. x glauca
In contrast to things like crops, they threaten bottom-up trophic cascade with their very presence. They need no management to accomplish this, and while doing so, they defeat that same diverse horde of enemies.
The success of these invaders kind of turns the whole “diversity is strength” thing on its head. Diversity may in fact be a weakness. @Marina_Gorbunova
I’d imagine the response will be something like “But those invasives became established due to human disturbance!” except that I don’t think that’s true in this case. Hydrology, floods, tides, fire, and other natural processes create more than enough disturbance.