That's an interesting point. There's certainly a cap on the complexity of code that can be put into a single long function. (Unless, I suppose, it has inner functions that call one another, like how people do OO in Javascript; such things can be just as complex as whole programs.) Usually it's implementing some conceptually unified thing. Even if that thing is a complex algorithm, it's still cohesive enough to be able to say what it is. And implementing even a very complex algorithm is not particularly complex at a system level.