I have some counter-thoughts about the quotation you cited. You felt the writer's points were self-evident. I am not arguing that the man's points are self-evidently wrong, but they certainly are not self-evidently true, and they, like your remarks, strike me as coming from a limited familiarity with the issues that make his statements more controversial than he knows. Let me respond to him in segments:
He is choosing these English words (among a larger possible list of plausible options) to translate aionios (he says) because of "contextual" considerations. If he were to go merely by lexical considerations, instead, he would have to admit that "endless" and "eternal" are not the only concepts (and possibly not the best-attested ones) that the Greeks associated with aionios. However, in selecting among multiple translational alternatives,context definitely matters. Therefore, I am interested in knowing whether the context of these passages really does yield "little doubt" that these meanings are intended...There is little doubt that contextually αιωνιον means "eternal," "endless," or "forever..."
It is true that the kingdom of God is endless. But does this necessitate that the alternative to entering it must be of equal duration? Suppose we were to say, "The twin babies did not share the same fate at birth. One was born healthy and lived a long life; the other was still-born and never experienced life in this world." We have described to opposites—live birth and still-birth. Yet, it is not obvious that the healthy baby's long enjoyment of life was of the same duration as the conscious experience of the baby who was deprived of such a life....since the state contemplated in Gehenna in the sayings of Jesus is contrasted with "entrance into life," or being in the kingdom of God, which shall endure forever, or eternally.
It is a point to be investigated further (not here—at least not by me) whether being deprived of an eternal privilege must result in eternal conscious knowledge of that deprivation. It is at least possible to envisage a hypothetical scenario in which one group of people were granted eternal life, and another group did not receive life, and perished as a result, without importing the unnecessary detail that those who did not enjoy eternal life must experience an equally eternal misery.
Support for the dichotomy that dictates, "Either it must be eternal joy or eternal torment" might conceivably be drawn from scripture, but not from logic. Yet it is just such flawed logic (i.e., a false dilemma) that informs the statement that there can be "little doubt" of this man's conclusions.
There are too many unfounded assumptions here. For example:The New Testament views "entrance into life" or the kingdom of God as entering "the age to come" (αιων μέλλον), the new and everlasting creation representing the final order of things. Thus a strong case can be made in support of the view that suffering in Gehenna will be endless or everlasting.
There is the assumption here that all people of all times are facing two alternatives: gehenna or salvation. But the fact that this was so for the original hearers does not, in itself, tell us whether the same would be true for later readers. Assuming that gehenna is the ultimate fate of unbelievers of all times is begging the current question under discussion. In other words, this identification is the very point at issue. If gehenna is a reference to the judgment of Jerusalem, in AD 70, then it was only for those people at that time that the failure to be saved would land them in that particular crucible. It might still be true that all people who reject Christ must face a horrendous judgment in a lake of fire, but that would not have any necessary bearing on statements about "gehenna" (which would not be the same subject as that of the "lake of fire").
There is the assumption that "gehenna" and "entering the kingdom" are being presented in these passages as strictly post-mortem options, rather than immediate and impending options for the original hearers. The assumption seems to be being made that "entering the kingdom" is what happens after the judgment day, when, in fact, entering the kingdom is what every person is called to do the moment they hear the gospel. Jesus' listeners were on the threshold of passing immediately into Christ's kingdom (by accepting Him), or else being cast into gehenna (by rejecting Him and facing, instead, the grusome holocaust of AD 70. This last statement is clearly true, regardless how we come down in the present inquiry. What is at issue here is whether the statements concerning gehenna are a reference to this immediate and near-term crisis, or whether they refer to a post-mortem condition that will prevail in the New Earth.