Granted, the reason the young man was excommunicated in 1 Corinthians was for sexual immorality. (Still a good reason for church discipline, IMO, since we're as wayward as ever.)
But from Paul's response in 2 Corinthians, the man's penitence resulted in the command to love him and receive him back into the fellowship, speedily, so as not to overwhelm the penitent with sorrow.
So, actually, if the young man had been penitent as soon as he'd been "caught" or admonished, one could assume he would not have needed excommunication.
Excommunication, then, is actually for the impenitent sinner.
Consider: If the elders or other brethren approach the acknowledged and confirmed sinner in a proper format for church discipline, if they ask him or her in firm holiness and Christian love to cease doing X sin, because X sin is bringing shame to the person and doing harm to the household of faith; if the person is then sorrowful and repentent and willing to cease the sinful activity and seek wise counsel within the church, taking advantage of whatever spiritual aid is available for reconciliation; then there is no need for the full disfellowshipping that is excommunication, is there?
So, impenitence is the real, sole sin that leads to proper excommunication.
Do you agree? Or do you disagree?