Why might my betta have died?

So I do have several bettas. My oldest and the one I am talking about was Kai. I believe him to be about 5 years old. I bred him once and he has always been kept in excellent water conditions. Two years ago he nearly died over winter as his heating pad stopped working overnight, but he survived. He has learned tricks but I retired him from those after I bred him back in May. The past couple months, his fins grew darker and darker. He was cellophane and in the end had a light brown color in his fins. He stopped flaring and ignored any other fish I put next to him. He started laying around and even ignoring me, even though he always used to be so happy to see me amd he would dance in greeting and follow my figer. Over the past two or three weeks he stopped eating, even when I offered him live bloodworms or his pellets or flakes. I realize he was old, but I have done nothing different with him and he was so healthy and happy and well cared for since I got him 4 years ago, so I want to know why he may have just started aging so abruptly and stuff. It is very sad, but I have seen his death coming for a few weeks now. I have his 21 week old young heir(ess) now. He was the fish that got me into fish keeping and betta breeding, so I am sad to say goodbye. Any info at all? And do you think there is a way I can encourage his baby to live longer than him? That baby already eats live foods as well as pellets and flakes, and fasts once or twice per week. I feed it as much as it wants until it stops asking. I have three other adults, two of whom I am conditioning to breed. The baby's mother also just died, but I think she got a bacterial infection in her fins because a tank mate bit her fin pretty low. I removed her when I found out the tank mate was too aggressive. All of my living bettas are all by themselves right now, with mystery snails.