Ignoring the influence one person can have is equally troublesome.

Suppose Constantine never converted to Christianity. Suppose Genghis Khan had been injured before conquering Asia. Suppose Louis the XVI had reformed the French government. Sure some other ruler would probably have embraced Christianity or some other ruler would have conquered Asia but history is fundamentally a sequence of events and minor changes, like a strong ruler in one place or another, can have massive downstream impacts.

>Suppose Constantine never converted to Christianity

What about the changes in wider Roman society that allowed for Constantine's conversion to have a large impact?

>Suppose Louis the XVI had reformed the French government.

Would those reforms have been carried out by the nobility? Would it have been enough to satisfy the peasants?

Focusing on "great men" is a far too simplistic view of history and it ignores the impact that each of us have on history as individuals.

"Great men" create Focal Points[0] (or, if you believe in great men, Schelling Points) around which societies and lines of history converge. It's probably equally true, though, to say that societal dynamics have within them focal points which allow certain men (and women) to become great.

[0] https://en.wikipedia.org/wiki/Focal_point_%28game_theory%29