I think that the actual process of sealing away Demise took away her divinity. Far from needless, she needed to keep Demise at bay until Link could come and kill him. Now, to be fair, Link seemed to have no trouble at all making a new seal with the Master Sword not even at full capacity. But then that was only for The Imprisoned, Demise's far weaker self. And the seal only held up for short bits of a time each. So either the specific seal Hylia used was so demanding that it literally required her to lay down her immortality to pull it off, or she was running out of time and options and this is the best she could come up with under pressure.
In the story that Zelda tells you in the past, she says that the goddess was injured a little too badly in her fight with Demise and would never have the power to stand up to him in the same way again.
After a long and fierce battle, the
goddess, Hylia, succeeded in sealing
away Demise.
However, soon after the demon king
was imprisoned, it became clear that
the seal would not hold long against
his fearsome power.
Hylia had suffered grave injuries in
her battle with the demon king.
She knew that if he broke free again,
there would be no stopping him.
And if the demon king were to free
himself, it would mean the end of the
world for all beings of this land.
In order to put an end to the demon
king, Hylia devised two separate plans
and set them both into motion.
First, she created Fi. She made the
spirit that resides in your sword to
serve a single purpose: to assist her
chosen hero on his mission.
Her second plan...was to abandon her
divine form and transfer her soul to
the body of a mortal.
...She made this sacrifice, as you have
likely guessed, so that the supreme
power created by the old gods could
one day be used.
For while the supreme power of the
Triforce was created by gods, all of its
power can never be wielded by one.
Knowing this power was her last and
only hope, the goddess gave up her
divine powers and her immortal form.
A couple times in the story it is mentioned that a deity cannot use the power of the triforce. However it is assumed that a demon can or else Demise would have no interest in it. So Hylia gave up her divine form to be capable of using the triforce against Demise, which Link later did himself while Zelda was sleeping in the past. If Ghirahim did not find the other gate of time and go to the past as well the plan would have worked out. Only with Link making a wish on the triforce instead of Zelda.
The only questions that this leave open are, why did she not trust any hero in the distant past or the present to make that wish. Fi mentions a few times during the triforce quest later on in the game that the triforce does have the power to defeat Demise once and for all.
Even Zelda mentions that only certain people can use it, and then states that Link is one of those people.
If you can obtain the actual Triforce,
we will have the power to vanquish
Demise once and for all.
The problem is, among the countless
souls in this world, only a select few--
those with an unbreakable spirit--
can wield its might.
To face Demise and give the land hope,
the goddess, Hylia, needed someone
with an unbreakable spirit. That
someone is you, [Link].
But spirit alone wasn't enough.
You had to overcome many trials and
awaken the hero within yourself so that
you could wield that supreme power.
And so Hylia... I mean, and so I...
I knew that if it meant saving Zelda,
you would throw yourself headfirst
into any danger, without even a
moment's doubt...
I... I used you.
can't begin to tell you how sorry I am
for pulling you into all of this,
[Link].
But you have to understand this is a
war, and the fate of the land hangs in
the balance. I need your strength to tip
the scales in our favor...
This makes it seem that Link was important to the plan all along from the very beginning. Hylia knew that there would be that one individual with the unbreakable spirit born a few thousand years in the future. So she used herself as the method of prodding him into going on his quest. Not to save the land or stop any demon, but to rescue his childhood friend from danger. This might be why Impa did not allow him to join them on their journey across the land below even though she stated he was the protector and chosen hero. She could have known about this plan all along as well and also helped move Link in this direction, strengthening him, his spirit, and the goddess sword.
However it might also be possible that not anyone off the street has what it takes to make a wish on the triforce at all. That the
unbreakable spirit is something that is earned or trained over a long period of time through many hardships. Zelda mentions something like this in the past as well.
Valiant hero, you have endured many
hardships and journeyed far in your
quest to reach this place.
Along your travels you have found
wisdom, power, and courage, and for
this I shall bless your sword with the
goddess's power.
May it give you and your sword the
strength to drive back the abomination
that threatens this land!
Now this again causes one to wonder why would she give up her divinity just to be that damsel in distress that persuades the hero into doing the right thing and making a wish on the triforce and stopping Demise. It might also be a part of her plan, for the hero to face Demise and defeat him anyway regardless of outcome. If the plan was just to make a wish on the triforce and kill Demise then there would be no reason to amplify the power of the goddess sword, or bless the Master Sword with the power to defeat demons all along. The wish would have accomplished that. Or in her amazing ability to plan for every possible outcome or just stack all odds into her master scheme by forcing all the pieces into place, she might have planned on having the hero with a special sword that is fully capable of exterminating demons face a very depowered Demise on a more equal footing and stop him for good. and all events were perfectly according to plan, which would have required Demise to steal some of the spiritual energies from a mortal and weakened Hylia to make a weaker than normal form that a hero could defeat.
Another thing to consider was that Hylia was injured in her fight with Demise. Possibly so badly that she might have been close to death herself. It may be possible that she could have survived but then she would not be anywhere near her previous level of power, not enough to be capable of being of much use if Demise were to return. Not powerful enough to stand against Demise, but devise so could not use the triforce either. So all of the plan was put into place because she would not have been alive at the time Demise's seal weakens enough to allow him to escape. So placing herself into a reincarnation cycle allows for her to be capable of taking part in resealing or even defeating him at a later date.