In I Corinthians 15 Paul speaks about vain preaching, vain faith, and vain hope. It is noteworthy that vain hope implies that Christ died in vain, i.e to no avail. This is reiterated by Paul in Galatians 2:21 "I do not frustrate the grace of God: for if righteousness come by the law, then Christ is dead in vain". Clearly Paul is saying that if righteousness is achieved by obedience then Christ died in vain. The prescribed path to righteousness before Christ was obedience. If Christ died to perpetuate that system, then what was the purpose of His death. If it changed nothing, then it most certainly was in vain.
Is it conceivable that Paul accepted the inevitability of Christ having died in vain in any sense? Christian theology would answer this question- yes! For everyone who fails to obey, Christ's death is ineffective; it was unsuccessful and useless.
Why did Paul struggle so mightily to denounce practices which threatened to make Christ's sacrifice a vain effort, if the very nature of God's plan made that sacrifice largely for naught. Such vanity resulting from a divine plan makes no sense, does it?