One does not have to ascribed to the Great Man theory of history to know that individuals do matter. Yogi Berra is said to have said, when you come to a fork in the road, take it. Two roads diverged in a yellow wood, but do all roads end up in the same place? Would McClellan have been a better choice in 1864 than Lincoln? We will never know. But likely things would not have been the same.
I agree that a single man can change history, but then we all do. And the greater the mob behind them, the greater the change. Hitler is a good example. And I do believe that had we not had Hitler, we may have had a war with the USSR around that time. That’s the problem with the “what if C=McClellan had been elected” question. Whatever happened, he would have been the “great man” that led whatever happened, e.g. allowed the seccession of the south.
And then during WWI we may have had an American front, where the north supported Germany and the South supported th brits/french. We don’t know. But without the conditions under which the “great man” got his support, he would not have been “great”. Europe was RIPE for a Hitler. The automotive industry was RIPE for a Ford. The computer tech industry was RIPE for a Jobs. Without that ripeness, we would not have heard of any of those men. Hitler’s art was atrocious.