Jesus said, "I will destroy [this] temple, and no one will build it [...8-9 letter spaces...]."
Isn't this a late reference, post-70 CE? If it is present in the Kernel, why do I date the Kernel to 50 CE (or earlier)?
The date of the Kernel was determined upon examination of the eschatology and the christology of the speeches, which reveals pre-Quelle traditions matching those that can be reasonably reconstructed and are indicative of the early Jerusalem church. The eschatology is such that the End is inaugurated and urgent with no sense yet of a delay. The christology is that of prophet and exalted angelic judge prior to the introduction of the title Son of Man. All of this is reconfigured (and rewritten) in the Gospel of Thomas over the ensuing decades, as the delay becomes realized and retrospective thinking about Jesus blossoms.
Saying 71 has always appeared to me to be a straightforward reference to the fate of the Temple at the End of time. The opinion that such a saying could only be explained in a post-Jewish war context is nonsense, and does not take into consideration the rich Jewish expectations about the Temple at the End of time - its destruction, either temporary or permanent.
Saying 71 is ticky though because we simply do not know how it ended because of the fragmentary nature of the manuscript at this point. All attempts so far at possible reconstructions have been for naught. It is impossible to know how it ended, although rebuilding it in three days cannot fit the letter space.
I see no reason, however, to think that a "prophetic" saying of this type could not have originated with Jesus himself, given the political and religious climate of the time and the "known" fate of the Temple during turbulent periods. I never like to speculate about Jesus' own words, but the "authenticity" of this particular saying is about as good as it gets. Bear in mind how Matthew, Mark, and John labor to revise it in light of Jesus' death, suggesting that the saying referred to Jesus' body, its entombment in the earth for three days, and its resurrection on the third day, NOT the actual destruction of the Temple.
I think that the canonical reference to rebuilding the Temple in three days smacks of Christian theological revision of a saying that either embarrassed the Christians or was unpopular. Since the Temple did indeed fall, the account that Jesus predicted its fall certainly would not have been embarrassing, but it very well could have been unpopular. Although the Jews and Christians toyed with the possibility that the Eschaton might bring the destruction of the Temple with no hope of restoration, this idea was not the favored expectation. It remains even within contemporary Judaism and some forms of Christianity that the destroyed Temple will be rebuilt.
So it may be that the Gospel of Thomas is preserving the oldest, harshest, and least popular remembrance of this saying, that the Temple would be destroyed unconditionally. The antiquity of its unconditional destruction appears to me to have been known to Luke too, who refers to the tradition twice but does not quote the saying itself (Luke 21:5-6; Acts 6:14).
These reflections can be found on pages 226-227 of The Original Gospel of Thomas in Translation, the commentary created as supporting documentation for Recovering the Original Gospel of Thomas.