According to the Order of Mata Nui, it was not up to them to question destiny, as decreed by the Great Spirit Mata Nui.
If a Toa team failed to achieve their destiny before their deaths, that destiny would not be accomplished by someone else. Every Toa had a destiny that was theirs and nobody else could do it in their place.
Destiny was not transferrable. If a Toa team died before their destiny was completed, the destiny would not get accomplished.
A Toa's ability to sacrifice their power before completing their destiny would depend on whether their destiny involved losing their Toa power.
Neither Mata Nui nor the Great Beings set destiny; it was a force beyond any single being's control.
It was possible to miss achieving one's destiny. If Vakama ignored Lhikan's request to bring the Toa stone to the Great Temple, for example, Vakama wouldn't have become a Toa even though it was his destiny.