Hmm... the NIV and NASB have "mature" here, not "perfect". In English, they are very different concepts.
Stretched beyond the breaking point. It is unreasonable to believe that Paul's second letter to the Corinthians contains the explanation for a concept presented in his first letter. You have made the error of anachronism. Further, you're making the argument that Gentile Christians (who are probably unfamiliar with the Old Testament scriptures) would know and understand an oblique reference to an obscure passage in the Torah.
For your argument to be sound, it must be cohesive, congruent, and complete. You've failed to address the incomplete knowledge and to identify either the "imperfect" or the "perfect" adequately.
Verse 8 notes the pending cessation of knowledge, prophecies (plural), and tongues, all of which are gifts presented in the latter verses of chapter 12. These are all revelatory gifts, and it makes "perfect" sense that when there is no more need for revelation, then revelatory gifts will cease. Are we there yet? No.
If we had full knowledge (as we are fully known), then we wouldn't need to study Scripture. We wouldn't need preachers, and we wouldn't need to debate the meaning and application of Scripture. We still know in part, and still need the revelation that (at times) comes through study and prayer.
We don't yet see face to face. We aren't yet fully mature. We will get there, but not in this season. God still gives revelatory gifts including tongues because His church still needs them.