Question: If the perfect of 1 Corinthians 13:10 refers to the completion of God’s revelation to the church, and thus prophecy is done away with the completion of the canon, then how can it be that in Revelation 11 the two prophets are inarguably prophesying?
Answer: The context of 1 Corinthians 12-14 considers three kinds of manifestations of the Spirit (12:7) in the body of Christ: gifts, ministries, and effects. Paul’s larger point is that these are all given for the common good (12:7), and that without love they cannot achieve their purpose (13:1-3). Love and the building up of the body is the point. The listed manifestations of the Spirit are significant means to that end.
The key point as it relates to this particular question is that the manifestation of the Spirit is given (hence the loose references to the manifestations as gifts) within and for the body of Christ. The context for spiritual gifts is exclusively the body of Christ. Consequently, the idea that the completed canon does away with prophecy needs to be slightly qualified: it does away with prophecy as a partial-revelation spiritual gift in the church. Paul is not discussing prophecy outside of that context.
What we see in Revelation 11 is a different context altogether. The two witnesses are indeed prophesying (Rev 11:3), and are even dealing in the realm of signs and miracles (11:6). But it is very notable that their ministry is a testimony (11:7) to the peoples, tribes, and tongues (11:9), and is considered by the people on earth to be torment, not edification (11:10). The church is no longer on earth, not having been specifically referenced after Revelation 3, and the church won’t return to the earth until Christ does in Revelation 19:11ff. In short, the two witnesses are not church-age believers, they are not ministering to the church, and they are not utilizing spiritual gifts as Paul described them. The ministry of the two witnesses is more like the Hebrew prophets of Israel’s monarchy period than anything we see during the church age.
So, the completed canon doesn’t end prophecy as a tool in God’s toolbox for use after the church age, but it does end prophecy as a partial-revelation spiritual gift to the church.