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.
So let me get this straight...
1. The Triforce can't be used by gods.
2. The Triforce was the only hope to defeat Demise.
3. Hylia became mortal
4. Link used the Triforce to defeat Demise.
...what does #3 have to do with #4? Couldn't everything have worked out just fine with #3 omitted? Link's not a god. He can use the Triforce. So Demise can be defeated.
But Hylia decides to sacrifice her divinity needlessly... just so she can waste away as a mortal holding The Imprisoned's seal. Why does she need to be mortal for that?
So, what's the point of Hylia's second plan? I think it's bad storytelling. They really wanted Zelda to be important, but weren't able to come up with a good enough reason for it.