More than 50 years has passed since the end of the Second World War, but this is the greatest disaster in human history.

More than 50 years 当然可以用作主语了。它跟 Five years, Three months 等是一样的。又如：

More than three years has passed. 三年多已经过去了。

Ten years has passed since we last met. 自从我们上次见面以来，已经过去10年了。