Why was the ritual needed (aka: why hasn't Finn just killed himself)?

Question

Please note that I'm not a native English speaker and I can't grasp 100% of what is said on the screen, so the answer to this question could be very trivial if it turns out that I've just missed an important sentence.

Even before the ritual is performed the original are all "linked" so the death of one is the death of all. So what's the point of having a ritual if the only thing to do was for Finn to kill himself, in order to kill all other originals AND all other vampires in the world (as explained later the death of a vampire wipes out all the related line)?

Also, at the beginning of "The Murder of One" Finn tells Klaus that he and their mother haven't given up, yet, even if the ritual failed. But, again, he only needed to kill himself (or allow the mother to do so) to kill all the others since they are all still linked at that point.

So what were they trying to achieve with the ritual?

Answer 1

At the time, the only other way to kill an Original, a stake made from the White Oak Tree, was believed to be gone forever. The ritual was a special spell that only Esther can do to turn the Originals back into human form so they can be killed.

From the episode "All My Children":

Bonnie: What’s the spell you’re doing?

Esther: As the witch who cast the spell that made them vampires, I can also reverse it. When they become human again, they can be killed. As they are linked as one, my brave Finn will be the sacrifice. With his death, will come theirs.

Esther cannot easily be brought back as far as the other characters know:

Esther: I draw from the entirety of the Bennett bloodline, living and dead. The connection affords me great power, although it is somewhat difficult to maintain, which is why I require you, mother and daughter. The bloodline made manifest.

Elijah: The witches that released my mother, she’s drawing her power from their bloodline, that line needs to be broken.

Breaking the Bennett line makes it harder for Esther to come back.

Aside from Esther's reversing spell, the Originals do not have any other weaknesses that can kill them besides the wood from the White Oak Tree. Later in "All My Children" Klaus and Rebekah find out that parts of the White Oak Tree survived but they did not tell Finn. Since Finn knows of no way to kill himself, Finn has no choice but to keep on living.