Jesus warns the disciples themselves about losing their salvation.
“I am the true vine, and My Father is the vinedresser. Every branch in Me that does not bear fruit He takes away”
John 15:1-2
You are interpreting Jesus' words with a predetermined idea that salvation CAN be lost. Note...Jesus does not say an unfruitful person will be sent to hell. What it certainly CAN mean is Jesus will remove an unfruitful person from: ministry, from having influence until the person repents and becomes fruitful again or Jesus will take an unfruitful person home (he dies so that his unfruitful actions do not cause shame to the name of Christ). Once it is understood that we can do nothing of ourselves and we must surrender our lives to Christ, so that HIS righteousness works within us, someone who may have been unfruitful before can, through the power of God, become a good and faithful servant of God. So, I disagree, Jesus is NOT warning his disciples about losing their salvation. Since salvation is not gotten by our works and merit, it certainly is not KEPT by them, either. That is what GRACE is all about.