I think that's the opposite of cooperation

No that's exactly what "cooperation" is, people passive-aggressively fighting each other for a bigger slice of the pie. The only way to cut through it is by having an benevolent dictator.

And what happens when the strong leader ages and dies or when he goes insane?

Is a problem that can occur. The fact that monarchy is sometimes dysfunctional doesn't absolve democracy of being always dysfunctional.

In my perspective ALL of human accomplishments were the fruit of cooperative efforts by groups of people

In each case pivoting on one critical figure.

Plus the entire scientific method is based on cooperative model. You have a theory and perform various experiments to test its validity; once you are sure about your conclusions you present your results to a worldwide audience who will be able to have further points of view to bring to the table, thus validating on an higher level or dismantling your theory

Which illustrates the distinction between the individual and the group. The group is incapable of innovation, it can only dismantle or attempt to dismantle. It takes the individual to innovate beyond the collective knowledge of the group. The individual is exemplary. The group is a herd of sheep.