Gangaputra Devavrata got two things from his father, Bheeshma name and Iccha-mrityu boon. But was that a boon really? As far as my view is concerned, it never acted as a boon, but a curse that was perhaps somewhere near the one Prabhasa received from Vashishta, due to which he took birth to live an extra long life full of sufferings.
So his life was destined to be longer than usual, but the Iccha-mrityu boon to had a part in the life he lived. I assume it would have been possible for him to die a natural death before the great battle in case he didn't have the Iccha-mrityu boon. He was the grandfather of the Pandava and Kaurava clan, but his age was equivalent to their great-grandfather as the first generation he saw was of his step-brothers who were more like sons to him, in terms of age.
Even if he would have survived till then and fought, he wouldn't have lingered breathing on the bed of arrows. The boon worked in this case, because nobody could remain alive on the bed of arrows, not even for the matter of a few seconds.
Bhishma remained alive just to fulfill the promise he had given to his late father. The physical pain is unimaginable! This Gangaputra conquered death in real senses. Shikhandi was just a defense, Arjuna was just the cause, nobody killed him, and he died on his own will.
So I seriously think the boon actually turned out to be a curse, it added even more to his sufferings, both mentally and physically. It's not necessary that a boon would remain a boon only. Time decides everything!
CID
