No, You were not overacting, but there is nothing you can do about it either. "The end is written in the stars." And for the most part, it is beyond human intervention.
As my granddaughter says, "Dang, that is too bad,...but now we can get chicks" The circle of life.
As for how long something should live...look at people. Really a lot of times, there is no rhyme or reason how long someone does live. The same thing is true of chickens. Just enjoy the lived ones today.
Some people do get real old chickens. They seem to get a lot of them. But I don't. I refuse to feel guilty for keeping my chickens in good food, good protection and clean water, and some of them die unexpectedly.
There are no guarantees, and as with all averages, well the ages of death for an average of 5-8 years can be 2 -12.
Mrs K