There isn't really a "true" meaning of any poem. It depends what type of analytical eye you are looking at it with.
In the sense of only considering the words within the poem:
I'm a little hindered as I can't find a clear definition of supersession. It seems to me to be that "death" as man knows it is something to be feared and thus has defined it by this. We hope to not die and for things to go well. Animals, on the other hand, seem to simply accept it. Therefore, man didn't create the moment when life ends, but what we call "death" due to how we define it and what we associate with it. We die every time something goes wrong because we have the feeling of everything ending and we dread it, but we hold onto hope, until the very end, that it will change. Then we repeat the process throughout our lives, seemingly causing ourselves more pain because we just don't accept events as events.
My attempt at analysis might be off due to the fact that I don't understand one of the words.