To me, Complexity, Effort and Time are three key things we need to consider. You look at the complexity of a story to derive the effort required to estimate the time it will take you to finish the story.
To use an analogy of carrying a log of wood from point A to point B:
COMPLEXITY: “How BIG and HEAVY is the Log of wood?”
EFFORT: “How much horsepower is required to pull this log?” This is where Story point comes in (IMO).
TIME: Say it takes 2 Horsepower, “how quickly can 6 horses move the log from A to B?” And this, we all know is velocity which can only be derived after a few Sprints.
Complexity cannot be seen in isolation and effort cannot be measured without knowing the complexity.