Why so harsh a treatment?
Long refers to this text as "provisions for people who have lost their way to be sought and restored, not forgotten." If I really try, I suppose I can discern some positives in this method of dealing with conflict between church members. It is persistent. It doesn't write off offenders. It values relationships enough to keep going back repeatedly to work toward reconciliation. It is focused on the restoration of the offender not revenge for the offended.
And, given the parable of the searching Shepherd that lies before it, I suppose we are to assume that the offended parties are in the right. The language of 18:15-20 is of regaining the lost, a parallel to the lost sheep parable where the Good Shepherd seeks and finds the lost sheep. Readers are to make a connection between the offended parties and the Good Shepherd. Church members who repeatedly seek out those who have wronged them for the purpose of reconciliation to the community are like the Good Shepherd who, in the parable that comes just before this, seeks out the sheep who has wandered astray. (I still see this automatic equation of church and Good Shepherd as potentially quite dangerous: In the larger context of Matthew's Gospel, we are warned of the need for constant humility and leaving the judging to God, themes that we must keep before us to avoid the constant possibility of self righteous judgments of others who are "not like" us.)
As for the harshness of the excommunication clause, I see that it is a last resort. And the status to which the church consigns such offenders (Gentiles and tax collectors) doesn't put them beyond the scope of Jesus' reconciling love. Gentiles and tax collectors are the very people Jesus the Good Shepherd seeks out to save and restore to community in his ministry. Even if the church separates from them, the Good Shepherd will still be out and about, seeking to find and restore them.
Then I come to the teaching on forgiveness with its ridiculously exaggerated math that follows this passage (18:21-22). The fact that Peter is the one who asks the question about forgiveness is a wonderful irony. We stand in this text looking ahead to Peter's threefold denial of Jesus. Surely, if anyone deserves to be ostracized and disciplined by the community, it will be Peter. And yet, we know that the Good Shepherd will forgive and restore him.
So placing this teaching about forgiveness immediately after the provision for exclusion seems to say that, yes there is a provision for leaving people to their own devices, for allowing them to be their own worst enemy, for ceasing efforts to reconcile with them . . . but don't ever view it as the last word. Forgiveness is the last word.
Naïve readings can't be the last step. They are the first. Why? Precisely because they are naïve. They need context, further exploration, further information. Naïve readings are like typing in comments on Trip Advisor without having visited the location about which you are commenting.
Thomas G. Long, Matthew (Lousiville, KY: Wesminster John Knox Press, 1997)