My oldest buff Orpington rooster is 5 this year, he is my oldest large fowl. When we catch and pick him up for spur and nail maintenance he gets all purple in his comb and wattles, which scares me. I'm sure one day he will just be dead. Most of my roosters go that way, so your thoughts on bad hearts might be the best answer for many of them.
I too really like bantam cochin roosters. They usually are all good roosters, but can be stinkers when young. Almost all of mine have spent months in rooster, but emerge as good little gentlemen. Mine even help raise the chicks, something only my oldest buff Orpington rooster will do too. I currently have 3 purebred Cochin roosters and 4 mixed slightly with d'uccle. They are also very entertaining.
My blue Orp Suede lived to be 5 yrs 8 mos old. I remember that day in July when his crow was weak. He just got worse, lost a lot of weight, slept all the time, on the ground or in the coop, then he passed in September, most definitely just old age. I think the really big guys, if they live that long, are quite lucky. It's sort of like big dogs are usually not as long lived as the little tiny ones (usually).