Always thought that was a load of crap myself. Gangplank sure didn't seem like he wanted to die and yet Illaoi was certain he was going to fail implying others who don't want to die still often fail.
Wanting to die and having the will to live are not the same thing. Living is not just surviving, it is moving your life forward. Gangplank was becoming stagnant. He was fine with just being king, and because of that he was shoved off the throne. If Illaoi was in his position, Queen of pirates and all that, that lady would of started her plans to take over the next area, and the next, and the next, and keep doing that until the day she died. She is motion, and her god(who, by the way, is real enough to take out your soul and smash it to bits) demands that all life be in motion, or to have their energy be given back to them so they can fashion something else that won't be quite as disappointing.
In other words, Necerabos says "use the energy I gave you well, or I will take it back". It doesn't care whether you give it homage or not, it wants you to live! Jinx, Gragas, and even Himerdinger are great examples of people who live according to Illoai's religion's code, yet they do not even know of it!