Well, we have to take into account that Bionicles are bio-mechanical machines: 1. Mechanical machines require energy to function. They also require an input where energy can be "plugged in," and an output where energy can be utilized. If any of those "input" or "outputs" were to be destroyed, how would energy reach vital mechanical parts of the Bionicle? Also, while energy cannot be destroyed, it is converted to heat (which is also energy, but that's irrelevant) after being used. This is why batteries do not have infinite power. The same applies for a robot. Bionicles do not have infinite power, and while they can "recharge," so-to-speak, eventually they will run out of energy. Since one could assume that a sort of "motherboard" would centrally run a Bionicle's functions and processes, we could also assume that that very motherboard would eventually lose its electrical conductivity (meaning it would decay). If the electrical conductivity decreases, the capacity for energy storage would also decrease. This runs in correlation with the idea of old age. The older we get, the slower our functions and processes work. Our brain loses processing power (after years of cell loss), the heart beats slower (there isn't enough energy to keep it and every other vital active at the same rate of that of a youth), etc. Similarly, mechanical parts decay over time. Assuming the laws of thermodynamics apply in the Bionicle universe, the metals that would make up a robotic system would eventually degrade making movement less viable and functions less probable. When Turaga Lhikan dies in MoL, his heartlight falters, and proceeds to go dead. In that sense, we can assume that vital mechanical parts, as well as biological parts, were badly damaged stopping energy flow throughout his body, and eventually causing his "motherboard" to go dead. 2. Biologically wise, very similar ideas coincide, especially the idea of decay. Biological organisms only have the energy to live for a set period of time. As vitals become frail, less and less energy is taken in, meaning processes function much slower. Take, for example, a computer. As parts get older, acquire dust, experience damaging conditions, they begin to lose power and conductivity and no longer function at the same speed they initially did. Biological organisms lose power and "conductivity" as they grow older and gradually slow down as well. -Rez