I presume that he was completely defeated because *runs for copy of LotR* Glorfindel says
Quote:
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Far off yet is his doom, and not by the hand of man will he fall.
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in Appendix A, part iv.
Doom usually implies that this is the end of somthing. I think that even if the Witch-King had survived, he would not do very well without his master, Sauron, after the fall.