Because of the millions of possibilities, this is why the Ancient One doesn't act based on what he sees of the future thanks to the Eye of Agamotto.
But in Ancestral's experience, some events or people are destined to happen.
One way or another.
Whether it's the force of fate that makes things happen, or as the only chance to prevent something much worse from happening.
One of them is his death and the succession of Stephen Strange as Sorcerer Supreme.
Stephen Strange is shown in many futures that the Ancient One saw him as one of, if not the best possible Sorcerer Supreme.
His ability to manipulate universal and dimensional energies is the best.
And his death is destined to happen to prevent a possible future in which Dormammu takes over his mind and wreaks havoc on Earth.
Ancient One has been able to harness the dark energy of Dormammu's dimension without his approval and without his knowledge to enhance his abilities and sustain his life, but this is only possible thanks to Ancient One's great skill.
Another sorcerer would lose his will and become a 'mindless', a puppet servant of Dormammu, just by coming into contact with dark energy.
One of the many events that will happen in one way or another is Dormammu's attempt to invade Earth.
In one way or another, a sorcerer or group of sorcerers, who don't necessarily have to be part of Kamar-taj, make a pact with Dormammu and allow him entry into the land.
Ancestral could certainly stop him.
While it is bound to happen that one or a group of sorcerers will attempt a ritual to allow Dormammu entry, it does not mean that one could not disrupt the ritual the sorcerers used to call Dormammu to Earth and deny her entry while she is alive.
After all, he has control of the earth's protections.
But seeing the future with the Eye of Agamotto is also a curse.
The Eye of Agamotto allows you to see millions of futures.
Whether the future is good or bad, for you or for others, it doesn't matter, the Eye of Agamotto will show you.
And Ancestral saw it.
A way to forever deny Dormammu entry to Earth.
Bound by an oath not to attempt to invade the land again.
Knowing that Stephen Strange would make a better Sorcerer Supreme than her, the Ancient One is determined that Stephen take his place.
But the time between Stephen Strange learning his mystical abilities and Dormammu invading Earth is very short.
Stephen Strange wouldn't have enough time to be a worthy Sorcerer Supreme.
Ancestral was in a bind as to what to do.
So when he saw a future where Dormammu was forced to take an oath not to invade Earth and Stephen Strange gained experience through clever use of a time loop, the Ancient One decided that so be it.
She couldn't be the one to trap Dormammu in a time loop.
Much of his strength comes from the use of dark energy, dark energy that comes from the dark dimension of dormammu
Although Ancient One is confident that she could withstand Dormammu in a head-on fight, she didn't want to risk Dormammu taking control of her and invading Earth.
So someone else will have to be the one to drive Dormammu back.
But here's another problem.
That person will need the Eye of Agamotto to achieve the time loop, and the Ancient One won't trust just anyone with its possession.
This person must be the next supreme sorcerer.
And the Ancestral's problem is compounded by Stephen Strange's decision to accept the position.
Stephen Strange is a stubborn and proud man.
Although his accident and subsequent learning of the Mystic Arts at Kamar-Taj gave him some humility, it does not mean that he accepts the burden of being the new Supreme Sorcerer.
And even if he agrees to be the supreme sorcerer, his mentality is not suitable for Ancestral to be comfortable giving him the position.
This is where his death serves a purpose.
Let's be clear: Ancestral isn't suicidal, nor does she have suicidal thoughts.
Even though he's lived a long time, it doesn't mean Ancestral wants to die.
But seeing that his death is necessary for Stephen Strange to mature and learn the importance of his job as Sorcerer Supreme, the Ancient One realizes that his death is necessary.
If the fate of the world is in his hands, Ancestral may sacrifice his life to safeguard it.
Having used the Eye of Agamotto extensively throughout his life, the Ancient One is very clear about what is most likely to happen in the next year.
And Ancestral knows that this book will have almost no chance of being found until some years later.
The chance of being found this year is slim, if not almost nil.
With this Ancestral knows that something has changed.
Something big changed so that the future will have changes years in advance of what she knew.
"Okay, I'm very happy that a lost book was found."
"Place it in the library so that the disciples and apprentices can read it."
"Oh, and also, tell Wong he did well to find such a valuable book."
Ancestral commented after reading the contents of the book and confirmed his suspicions of a change in the future.
The librarian received the book from Ancestral and bowed before leaving.
Ancestral remained seated, even long after the librarian left the room.
Her mind was in a mess, worried about the changes she would encounter when reviewing the possible futures with the help of the Eye of Agamotto again.
As mentioned above, the Eye of Agamotto shows both good and bad things that could happen, so it's understandable that the Ancient One might be a little worried about what she might find.
Knowing that no matter what he does, what he sees with the help of the Eye of Agamotto will not go away, the Ancient One stood up and headed to where he keeps the Eye of Agamotto.
Taking him out of the place, Ancestral sits cross-legged and manifests eldritch magic.
With the movement of his hand, light and sparks appear on the Eye of Agamotto, slowly causing it to open.
Little by little, a green stone appears from within the Eye of Agamotto.
The time stone.
Knowing the abilities granted by the Time Stone and his own skills and knowledge, the Ancient One was able to easily harness its power.
The first thing Ancestral did was not look into the future.
The first thing he did was look back.
If Ancestral were to look at possible future events right now, she could see the most likely changes in time; but it wouldn't be as accurate without knowing what caused this change.
So that's what he did first.
Find out what caused Wong to find the book the librarian had shown him years earlier.
Green lights appeared in his vision, and little by little the past of the room was shown.
In the vision, the Ancient One saw herself grabbing the Eye of Agamotto, and as if she were doing everything backwards, she was slowly backing away as if she were leaving the room.
Since this wasn't what he was looking for, Ancestral shifted his perspective to pursue what he wanted.
The librarian was quickly shown on his journey from the library to the meeting with the Ancestral.
Speeding up the vision, the librarian stepped back, showing what he was doing before.
Ancestral saw the meeting between the librarian and Wong, the son of his assistant Hemir.
Ancestral saw their entire interaction from start to finish.
From their search for the book together to the moment Wong arrives at the library and tells her about the old book he found on the Internet.
Since this vision didn't show him what had changed to allow Wong to find the book earlier, Ancestral continued to retreat.
Ancestral stopped as Wong was reading the message on his laptop.
Ancestral saw that Wong's demeanor changed when he read the message, causing him to grab his laptop and head to the library.
Knowing he had found what he was looking for, Ancestral shifted his vision's perspective to one where he could read the message.
Without having to move his hands and just by thinking, the perspective of his vision quickly changed and Ancestral found himself in the perfect position in front of the laptop screen and easily read the message.
'That's it, this message is what caused the change,' Ancestral thought only a moment later.
Now you just have to find out the origin of this message.