[Bakary]

Essentially it's a clash between the Great Man conception of history and the process version. The Great Man version is easier to understand. You can look at a specific individual and easily conclude that their actions had an enormous impact. For people such as Mao who had sway over billions, it is certainly a conclusion that seems to withstand quite a bit of scrutiny. But any person is a product of their context and we have to deal with multi-factored forces that might be impossible for a single human mind to model or grasp given the quantity of data. This is particularly relevant for scientific pursuits as opposed to political decisions. Newton and Leibniz sound irreplaceable if you read their biographies, but they came up with calculus separately around the same time. The same goes for Darwin and Wallace. If the conditions are ripe, individuals matter less. Technology isn't a predefined ladder like in the civ games, but every civ is at a juncture where so and so technology has a probability of being discovered. It's not unrealistic to assume that if certain lab conditions exist, it's only a matter of time until someone stumbles on to penicillin even if from a historical and emotional perspective it seems like a freak accident.

I can't draw a conclusive answer to these questions following the logical consequence of my own arguments, but at least we have to come at the problem with the knowledge that our own minds are drawn to simple narratives and to individual achievements. Hence assuming replaceability in the absence of very strong evidence to the contrary