Yes, it is a valid question. I certainly was not denying its validity, but I had thought that the verse answered it sufficiently.
The verse that you have quoted fits in with the theme of Matthew 7:17-18 perfectly. Those branches which do not bear fruit do not belong in the kingdom. They
never belonged in the kingdom. Read forward a couple of passages:
(John 15:5-6) I am the vine, ye are the branches: He that abideth in me, and I in him, the same bringeth forth much fruit: for without me ye can do nothing. If a man abide not in me, he is cast forth as a branch, and is withered; and men gather them, and cast them into the fire, and they are burned.
No. It means that their lack of fruit shakes their assurance. This is the warning of Hebrews 10:26-27.
For if we sin wilfully after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins, But a certain fearful looking for of judgment and fiery indignation, which shall devour the adversaries.
But the author goes on to say that such is not true of believers in v. 39:
But we are not of them who draw back unto perdition; but of them that believe to the saving of the soul.
Since an evil tree cannot bear good fruit and a good tree cannot bear evil fruit, any tree bearing forth good fruit is of the vine of Jesus Christ.
John writes about those among the church who go out never to return:
(1 John 2:19) They went out from us, but they were not of us; for if they had been of us, they would no doubt have continued with us: but they went out, that they might be made manifest that they were not all of us.
Though they were among us, they never bore fruit. They were never grafted into the vine of Jesus Christ. As unfruitful branches, they will be cut off and thrown into the fire. But those who remain and abide in Christ do so eternally.
Think about the prospect of "unelection." Is it reasonable for us to assume that God would
unchoose people after redeeming them? What would be the reasoning behind that? Would God admit that he made a mistake? That the person really wasn't "good enough" to justify election? Who among us is? That is precisely why it is called
grace.
On the prospect of unfruitfulness, we need only look to nature to complete the analogy. Some trees bear more than others. Some seasons produce more than others. Some seasons see no fruit. Sometimes there is blight and famine. Other times there is a bumper crop. Trees do not bear constantly. During the winter, they go into hibernation, preparing for the warming sun to return to touch their leaves that they may begin the cycle afresh.
But do not doubt, he that abides in Christ will bring forth fruit and that abundantly (John 15:5).