The prophecy that they will prophesy cannot be grounds for completion, because they have yet to prophesy. It is grounds for the exact opposite conclusion.
I don't really find such an option to fit with the witness of the scripture.
Even if their very prophetic message(s) could be read in your Bible right now before they prophesied it, that still would not excuse the fact that they have yet to prophesy it. This would still show 2 things: 1 Cor. 13 would still be unfulfilled because the act of prophesying has not been done away with.
When 1 Cor. 13 is fulfilled, there will be no more prophesying.