Revival,
Your argument is based on the false premise that we are inconsistent with our definition of aionios. Some may be, of course, but not all are. For instance, I don’t ever translate it “eternal.” But if you really want an essay on it, first, like all words, it has a lexical range. It can pertain to a period of time (“lasting for an age” or “characteristic of an age” or “befitting an age”). That’s the primary definition we use in Scripture. Zoen aionion is "life befitting the Age to Come and kolasin aionion is “correction befitting the Age to Come” and pyros aioniou is “fire befitting the Age to Come.” None of those, of course, means “eternal” in itself, not because such a thing cannot be eternal, but precisely because no length of time is in view at all. Of particular interest is Jude’s reference to Sodom; the fire that burned it up is definitely not still burning!
Now, on the other hand, aionios can refer to things that are more or less permanent. It’s used numerous times in the Septuagint of the OT to describe things like “mountains” and “hills” that aren’t going anywhere for a while. Obviously, in these circumstances it’s not a proper “eternity” in view, either, but in this case, it does pertain to length of time, whereas in the previous circumstance, it doesn’t. Once again, the word does not actually preclude complete permanence/eternality; it simply doesn’t require it.
Aionios can, in some literature, refer to the idea of being “properly” eternal. In classical Greek it held this meaning. Plato used it to describe a timeless reality, without future or past, but among Koine speakers, that understanding is exceedingly rare, and for first-century Semitic Koine speakers, that idea would have bordered on nonsense unless applied directly to God Himself (on the other hand, for a classically trained, Gentile Church Father some centuries later, such as Jerome or Augustine, that idea would have appeared quite sensible).
To boil it all down, there is a qualitative sense of aionios that we find in most of its usages in the New Testament. Clearly zoen aionion is not about the length of life, but about the kind or quality of it–the kind of life that can only come from God, that characterizes the Age to Come, etc. On the other hand, kolasin or olethron aionion ought to be taken the same way: not about the length of time that the correction or destruction will last, but the sort of thing it is. It’s the kind of correction or destruction that can only come from God, that will mark out the Age to Come. It is the Correction of the Age.
Now, earlier you cited John 6:47, asking if it meant “everlasting life” or “life of God” or what. Let me suggest N. T. Wright’s rendering, noting also that he is not only a passive non-Universalist but has actually written against UR:
“I’m telling you the solemn truth,” Jesus went on. “Anyone who believes in me has the life of God’s coming age."
Why did he render it like that? Because, obviously, the point isn’t how long the life lasts; it’s the kind of life it is. There are other words to suggest that it lasts forever, such as “immortal”–words never applied to punishment in the New Testament (although, curiously, applied to punishment by Jews that believed in eternal torment outside of the New Testament). Scholarship is only beginning to shake centuries of tradition here. There’s a reason why Clement of Alexandria, a Koine Greek-speaking Christian of the second century, did not hear “eternal” in aionios while Jerome, a classically-educated Christian of the fourth century, did.
We ought to be grateful that none of the words that always properly means “lasting forever” is ever applied to eschatological punishment; if it were, we would have a clear-cut contradiction between the Old Testament Prophets who say in no uncertain terms that God does not punish forever and other (hypothetical) passages that say He does. As it is, we don’t have such a contradiction because none of the passages regarding eschatological punishment are required to be understood in terms of it actually persisting forever.