How and why is Dickinson's "Because I could not stop for Death" structured as an arc?

A narrative arc consists of an introduction, rising action, a climax, falling action, and a conclusion. A character arc occurs when a character undergoes some kind of change in the story or poem. This change could be external or internal (mental). 

This poem has the visual structure of an arc in a general way because Death takes the narrator on a journey. They ride from one place to another.

There is a debate about what...

A narrative arc consists of an introduction, rising action, a climax, falling action, and a conclusion. A character arc occurs when a character undergoes some kind of change in the story or poem. This change could be external or internal (mental). 

This poem has the visual structure of an arc in a general way because Death takes the narrator on a journey. They ride from one place to another.

There is a debate about what Dickinson implied in terms of an afterlife in this poem. There is no definitive indication that the speaker/narrator is headed toward a heaven or some afterlife. But there is also no evidence that denies this possibility. What does seem certain is that the speaker comes to realize that death (and/or the afterlife) is eternal. This is the internal change the speaker goes through in the poem. The arc takes a visual form of the journey. But the character arc is explained by the speaker's realization that life had been short and that death is forever. The carriage holds her, Death, and the chaperon, Immortality. The speaker has been dead for centuries but it has felt less than a day. She realizes what death and eternity really are. Life exists for a time, but death lasts forever and therefore, time (in death) is irrelevant. 

The speaker goes through an internal (mental/spiritual) realization and this move from her initial death to the awareness of its eternal duration is her transformative arc. One could also argue that the narrative arc consists of the introduction of the speaker dying (meeting Death), the rising action of the journey, the climax (realizing the eternal), and the falling action and conclusion of this deceased speaker reflecting on the entire journey. (The fact that the speaker reflects on this centuries-long journey suggests an afterlife, but this could also be Dickinson's abstract way of describing the eternal nature of death.)