The true (actual, Marxist red pill) answer: work is exploitation of laborers by capital and "not good enough to be worth employing" isn't really a thing. The bosses make off with so much money, who cares. Maybe there are a few useless people at the bottom sucking off salary. What about the useless ones at the top, though?

All this being said, in the context of an organization that actually deserves to exist (a rarity, given that we live under capitalism), it is impossible to say what percentage of people will be ill-fit to their roles, let alone what percentage will be so ill-fit to any role that separation is the only option.