I agree that there are many types of "enlightenment." Even in the East they differentiate between a satori, samadhi, nirvana, nirbikalpa samadhi etc.
I think most people are not talking about the simple understanding or learning, which is one of the definitions of the word, when they discuss enlightenment in spiritual or mystical contexts. To be enlightened in this sense seems to imply a somewhat permanent upward shift in consciousness... a kind of spiritual quantum leap. When an electron gathers up enough energy it can disappear from the ring it is currently rotating in and reappear in another higher one. There seems to be this kind of conception regarding one's consciousness.
I have met a number of people who's range of consciousness is significantly higher than that of "normal" people to the point where their most unconscious states are already higher than the high points for a typical person. If someone is so elevated that they never descend into petty egoic bullshit, maintain peace and equilibrium in all things, and are continually able to maintain states that normal folk need a hefty dose of entheogens to approach... I consider these people "enlightened masters." They are rare, and it is an honor and blessing to be around them.
Most of us on the path to mastery lie somewhere between the "normal" ego attached, going through the motions, auto-pilot masses... and the true masters. For us, we may taste some of their bliss in moments we can consider satoris or revelations, but be unable to maintain such exhalted states indefinitely. We might require lengthy practices, entheogens, or other shamanic tools to get up there, and suffer the indignities of falling down into petty BS from time to time. On this path, we may initially measure our moments of enlightenment in milliseconds.... brief pauses between breaths where we "white out" momentarily and grok the nondual. We may have entire half hour forays into ecstatic bliss when boosted by ayahuasca or some other such tool. People who experience these things are not fully enlightened, but they may very well be on their way.
What I notice is that sincere seekers of truth can reach out and touch it in timeless moments of ecstasy, but this doesn't guarantee that they will be able to maintain such lofty and rarified states. What it does do, is acclimatize them to those heights. After a while, what seemed impossibly cosmic and unreachable becomes somewhat repeatable and familiar... not an immersion into a foreign and befuddling landscape, but a trip to an exotic but known locale where you start to know your way around.
The range of your experience moves up via such pathways. You still have your highs and lows, but they are all moved up so that your lows are no longer quite so low... and your highs are, well... higher. When you reach a state where your lowest points are qualitatively higher than your former norms... you can consider yourself somewhat of an adept. When your lows are higher than your former highs... you are probably worthy of considering yourself a master.
Of course, I have never met a master who was foolish enough to think that they were done, or that there was nothing more they could learn or achieve. Even if you had the experience of knowing all there was to know, the Universe is not stagnant. There are always more subtle nuances and finer levels of control. Better retention... better integration... a sharper intent. When things you struggled for in the past become nearly effortless, this doesn't mean you are done... just that you have new mountains to conquer.
A person might have a masters degree, and still have a lifetime of study ahead of them... but that doesn't mean that they aren't lightyears ahead of a bunch of kindergarten students.
All that said, I do think that total enlightenment is a reality. Not just a possibility, but a certainty... for all of us. The you at the end of time is what I call the "highest self," and this being is fully realized and transcendent IMHO. This is the future version of you who actually knows who you really are. Not intuits, not grasps, not momentarily recognizes... but thoroughly and completely knows.
Be well friends,
HF