Warning passages

In Scripture, there are all sorts of warning passages which seem to indicate that a true believer in Christ can forfeit eternal life. Though there are many that could be cited, this one is pretty strong:

““Every branch in Me that does not bear fruit, He takes away; and every branch that bears fruit, He prunes it so that it may bear more fruit.“You are already clean because of the word which I have spoken to you.“Abide in Me, and I in you. As the branch cannot bear fruit of itself unless it abides in the vine, so neither can you unless you abide in Me.”
(John 15:2–4 NASB)

Can one who is "in Christ," a person that produced good fruit, be "taken away" due to the fact that his works (or lack thereof) showed that he is no longer a follower of Christ? This verse would seem to indicate such.

But could there be another option? Is it possible that these warning passages serve a means to an end? In other words, the "ends" being the promise of eternal life (John 5:24) and the "means" being the warnings that unless you are bearing proper fruit (John 15:2-4), you will be "taken away" from Christ. Some might argue that this is not a possibility, given the explicit warnings in passages like these. If this argument is made, then one wonders what to make of the following:

““Yet now I urge you to keep up your courage, for there will be no loss of life among you, but only of the ship. “For this very night an angel of the God to whom I belong and whom I serve stood before me, saying, ‘Do not be afraid, Paul; you must stand before Caesar; and behold, God has granted you all those who are sailing with you.’ “Therefore, keep up your courage, men, for I believe God that it will turn out exactly as I have been told.”
(Acts 27:22–25 NASB)

Here, Paul provides a promise to those aboard the ship: they will not die. Consider the parallel:

““Truly, truly, I say to you, he who hears My word, and believes Him who sent Me, has eternal life, and does not come into judgment, but has passed out of death into life.”
(John 5:24 NASB)

"Yet now I urge you to keep up your courage, for there will be no loss of life among you...for I believe God that it will turn out exactly as I have been told." (Acts 27:22, 25 NASB)

Here, we have one spiritual promise and one material promise. The parallel is that both guarantee something.

However, in the Acts 27 account, there is something else we need to be mindful of:

“Paul said to the centurion and to the soldiers, “Unless these men remain in the ship, you yourselves cannot be saved.””
(Acts 27:31 NASB)

Why would Paul issue such a warning after he guaranteed that they would be safe? What seems to be the only option here is that this warning is simply a means to an end. That is, the guarantee is qualified with a warning. Thus, in this case, the warning is not even a possibility, but simply an affirmation of what would happen if these men leave the ship.

Could the same apply to the spiritual? Could it be that the warnings are not as much a reality as they are the means by which God sovereignly preserves the elect?