The well is the final transformation from "normal person" to whatever it is the cultists are. The whole thing can be read as an allegory for Western culture's addiction to appearance and the modeling world's fascination with unnatural forms of "beauty." Check out this famous
video to see what I mean. The overt comparison is to a cult, with the slow indoctrination and separation from family and friends. This can be read both literally and metaphorically (i.e. actual physical separation and the mental walls created when putting our "best selves" in front of the world.)
The ending is the climactic moment when the change is irreversible; submersion in the acidic liquid apparently burns away everything that isn't "beautiful," according to whatever concept of beauty motivates the cult in the story. The impact is so strong because of the heavy emphasis on that last line: "Open your eyes." Remember, that milky liquid stings on the skin. Imagining something like that touching your eyes directly is almost guaranteed to evoke a reaction in a human; we're very anxious about our eyes (as well we should be, given how much we rely on them.) Instinctive flinching and protective gestures are very common when the eye is threatened.
On a metaphorical level, it is even more disturbing, because the eye is the seat of perception. It's how we take in the world around us, and what we learn about the world is, in a large and almost literal way, a part of who we are. If this fluid alters the very perception of the world for those who are bathed in it, then how will the world be different when the new cultist emerges? There can be no going back at that point.