Without the last three paragraphs, it wouldn't be a Hawthorne story ... in fact, it wouldn't have much of a point at all. The whole idea is that, even though he doesn't really know anything for sure, Brown is willing to suspect the worst of all those he has lived without any real "evidence." Many of Hawthorne's stories are based on ambiguity ... that things are not always as they seem, that people are not always as perfect (or bad) as we may wish (fear) ... some great insights into human nature.
Without those paragraphs we may have an interesting story (not for me), but we wouldn't have the masterly study of one man's discovery of every other man's humanity.
If the last three paragraphs were omitted, the reader would not know the reaction of Goodman Brown on the following day. If "Young Goodman Brown" ended with the dew besprinkling his cheek as he fell against the rock, (the end of the fourth last paragraph), the reader would have nothing but his/her own conjecture as to how the relationship of Faith and Goodman would continue.
Perhaps, the reader would find the cold dew symbolic and surmise that the marriage between Faith and Goodman also grew cold since Goodman was disturbed at his wife's attendance at the meeting. For, in the previous paragraph, Hawthorne writes, "What polluted wretches would the next glance show them to each other, shuddering alike at what they disclosed and what they saw!"
This observation is the only indication that the reader would have of the dissolution of Goodman Brown's faith in both his wife and in the community if the story were to end three paragraphs early.
One of the things that makes this story so memorable is the ambiguity the reader is left with at the end of the story. As Hawthorne, himself, asks, " Had Goodman Brown. . .only dreamed a wild dream of a witch meeting?" Hawthorne leaves the reader with a mystery to contemplate. However, for Brown, the ending would not change. He has lost his "
Faith" both literally and figuratively because he can never trust anyone close to him again. He is unhappy for the rest of his life and even without the last three paragraphs, Brown still looks "sternly and sadly" at his wife and passes her "without greeting."