^Well you were earlier trying to argue that the crystal may have had the power to increase Kuja's lifespan. Yet there is no evidence for that within the game. Is it not fair then to assume that the crystal couldnt extend Kuja's life?

Kuja knew a lot of stuff about magic, so it seems reasonable to assume that he might have known about the powers and limitations of crystals as well. I dont view 'Kuja not trying to use the crystal to increase his lifespan' as a sign that he lost the plot and went mad.