It's a complicated topic and I wouldn't want to launch into a huge debate about it, but basically...
Too late!
It contributed to a "stratification" of humanity. There weren't "mortals" and "the Exalted;" there were extras < mortals < heroic mortals < enlightened mortals. This in turn contributed to a warped perception of how characters "mattered;" being an enlightened mortal with supernatural martial arts, ideally Artifacts, possibly sorcery, was the ideal endpoint for any mortal, their potential achieved; any mortal below that was underachieving. This in turn contributed to a climate in which it was okay to dismiss mortals as irrelevant, because there was an upper tier of super-mortals who could serve as mooks for your Exalts.
This is because you completely misunderstood how the distinctions worked. Extras were not below enlightened mortals. It was, in fact, possible to be both at the same time. Merely having access to Essence channeling was not enough to make you a Not Extra. A pack of Blood Apes attacking your party were just as much Extras as a pack of brigands, just with higher base stats. A group of Enlightened Grass Ninja may be elite mooks, but they were still mooks and the rules would treat them as such regardless of how many Charms they could use.
Heroic was not a distinction of power level but a distinction of importance. A heroic character was a Named Character, one whose personality and choices mattered and who would get a speaking part. Again, this was unrelated to their ability to use or not use Essence.
Vanilla mortal was a third kind of distinction, one that meant the character wasn't unimportant enough to be considered an extra but also not important enough to be considered heroic. Thus while they got full health levels, they couldn't stunt since they were Gm controlled characters not meant to stunt or gain an benefit. They are recurring characters the players may care about enough that you want to apply the full rules of the setting to, but not important enough to justify fleshing them out that much.
Characters could even change roles between these three states as the game progressed. In the early game you might consider a Blood Ape a fully heroic character, especially if you are playing a game with a prelude pre-Exaltation or a low tier Dragonblooded campaign but as the game (and the PCs) progressed in power most you would encounter would become Extras.
Later into 2nd edition some of the rules began to treat these as proper distinction rather than storytelling tools for the GM. Charms and effects began to differentiate between them and suddenly the difference between them mattered a hell of a lot more. This was a problem with those mechanics being terrible, not with the rules for extras and heroic characters being broken. You suddenly had magical abilities which were explicitly
in setting effecting people in a way that was
meta-narrative level. It's like Charms that keyed off of Stunts, which was and remains a terrible idea. Stunts (and extras and heroic characters) are gameplay level concepts that should not be incorporated into the narrative level as things characters can consciously
choose to be.
Enlightenment looked very mechanical, very rational; it was possible to systematically enlighten mortals en masse through the proper channels. This led to a surge of schemes designed by players to mass-produced enlightened mortals, as they were the best mooks you could get; this turned the setting into a parody of itself, where instead of having a wuxia world of supernaturally endowed martial artist you had social (and literal) engineering aiming at fielding armies of Artifact-equipped, TMA-using faceless mooks. From this came the idea that enlightening all humans was a reasonable - in time, even standard - goal for a Solar/Infernal character, that this was helping humanity achieving its true potential.
See I have no problem with this because mass Enlightenment
should sound like a good idea... until you begin to realize how terrible an idea it is.
For example, the difficulty to weave Fate for a section of Creation is based on the Magnitude of the people that live in that area. So an area with a Magnitude of 10 had difficulty 10 to weave Fate for it. But this only counted for Unenlightened Mortals. Every being with Essence 2 plus was not counted as part of this Magnitude but instead added their Essence score to the difficulty. So, for example, a 10,000 person town would go from Difficulty 10 to Difficulty 20,000 if you Enlightened everyone.
Basically, the only beings capable of maintaining the Loom of Fate at that point would be the Maidens. And they're occupied.
So yeah, go ahead and turn six billion people into Enlightened mortals. Heck, the Fair Folk will help you for free.
There were other details. For instance, it made the gunzosha warriors of Lookshy into a joke; they were not heroic super-soldiers making a sacrifice of half their lifespan to use their Artifact armor, they were a bunch of chumps that no one had bothered to enlighten, as that would have removed all drawbacks of their armor.
@Fenrir666 is a friend and a swell person and this is not at all a blame I put on him but merely a natural consequence of the system as it exists, but the way he designed his Followers by going "oh, and I'll bump them 2 dots so they're all heroic enlightened gunzosha warriors" trivialized so many things at once, I still haven't gotten over it.
Well, even setting aside the stuff I pointed out up above as to why Dragonblooded would be encouraged in no uncertain terms not to allow Enlightened mortals to become too big a problem the trick to understand here is that Enlightenment wasn't supposed to be something easy and safe. The problem here was Solar Charms that we're basically "pay x to enlighten" which allowed you to set up factories to produce the buggers.
All you need to preserve stuff like Gunzosha is to introduce dangers and drawbacks to Enlightenment. Thaumaturgic enlightenment should be either incredibly dangerous and likely to kill you outright, or take so long that using it to produce viable soldiers is useless. Or it should have required dangerous vision quests in the Wyld, with all the dangers involved in that. Or careful breeding programs involving demons and elementals and forbidden gods, with all the dangers of that.
Your problem is with the
ease of acquiring the benefits, not the benefits themselves. So long as the costs are prohibitive enough, you won't see them used overwhelmingly.