So, why is a doctrine and procedure created based on only what Paul said?
One the one hand, one could see this as Paul speaking off his own bat. On the other, he could simply have been formalising what was a de facto situation/arrangement.
There is no record of any dissent from Paul's view on this, unlike in other areas where it is recorded that there were heated debates and votes by older men to decide what should be done.
I think the question revolves more around, not if disfellowshipping of some sort should ever happen, but what exactly it should involve.
As Fisherman said, there is nothing odd about the idea of expelling someone from a group if they refuse to follow its 'rules' and will not be a peaceful or orderly member of the group. That's normal in all sorts of settings.
However, the question comes with how that person should be treated afterwards.
Based on the Scriptures, it seems that that depends on exactly why the person left or was disfellowshipped in the first place.
In my opinion, when we compare what the scriptures say about how Jesus acted around his enemies, how we should treat family members including unbelieving marriage mates, and also the verses that Christians are encouraged to "exhort" the disorderly and the weak, it seems that the current JW policy of total shunning in all cases appears to "go beyond what is written".
In hinges much more on whether the disfellowshipped person has taken a stand against Jesus or become an "antichrist" or opposer, possibly introducing contradictory teachings about Christ's role, such as denying the resurrection. That is the context in which Paul says Christians should "not even say a greeting" to such ones, but not in every kind of case where someone leaves the congregation.
Again, the context also seems to show that "disfellowshipping" is much more about not inviting the person into your home for fellowship or sharing in worship with them, rather than literally acting as though they are dead, such as the way JW videos show parents ignoring desperate phone calls from children or vice versa.