Edit: I have to correct something here in this post which is that there is already a term that replaces my made up term "devoid theist." It would be "implicit atheism." Therefore, just replace my made up term with that one.
First off, I am going to explain something about atheism. When babies are born, I don't think we would even refer to them as atheists. Atheism is a disbelief in gods. But since these babies would not even have any disbelief at all since they are not even aware that there is no god for them to develop any sense of disbelief in the first place, then I don't think they would even be atheists. We might have to come up with a different term instead. I will just make up a term and call it "devoid theist."
But where I am getting at with this is that if all human beings were devoid theists, then it wouldn't make any difference if we were atheists. We would still be just as encouraged and motivated to live this one and only life. Therefore, I see no value in realizing that we are all just going to die and not live on in some afterlife. The only value here would be living life in of itself. A devoid theist would find just as much good value and would be just as inspired and motivated to live this one and only life just as an atheist would.
That being the case, it is only this beautiful and great life itself that holds value. Actually, I think people who are devoid theists do have this idea of living forever. If you are unaware of death, then the only thing you are aware of is living. So, in a sense, you would be aware of nothing but living on forever. Therefore, it is the idea of living forever that holds the greatest value for people who are doing just fine in life and are living their good lives. So the greatest life can only be defined as living forever in the lives that we personally deem as the greatest value for us.
But being aware of mortality holds no value. As a matter of fact, there are many people who are depressed, fearful, and have a sense of less good value and worth in their lives knowing that they will just forever die in the end. Why is this? It's not just because of some irrational thought these people are having. It is because fear and depression are natural stress responses. Depression, in addition to being a chemical imbalance or something wrong with the brain, is also a natural stress response to life's stressors. It warns us when something is wrong in our lives that hinders our survival.
Since death obviously hinders are survival, then it is only expected and natural for many people to feel depressed, enraged, fearful, etc. about the very idea of dying even if it is somehow an irrational stress response. If you were told that by tomorrow you would die from some illness, then it is only natural and expected that you would feel depressed, enraged, fearful, etc. just as natural as it would be for an atheist to also feel depressed, enraged, fearful, etc. about dying. People who don't feel this way just simply means that their minds are wired differently and just aren't bothered by the idea of dying as an atheist.
Therefore, since being a devoid theist would bring us the greatest lives, then it is mortality that is the problem here since it makes so many people depressed and such. Even if there are many people who just get over that stress, mortality is still a problem for many people. Therefore, it needs to be eliminated through a cure in the future.
Animals all live just perfectly fine not being aware of their death (limited lifespan). As a matter of fact, if one of these animals were to be somehow suddenly aware of their limited life and that they are going to eventually die, then that would only serve to cause them depression, fear, rage, etc. So just as it is with them, we are also better off not being aware of mortality. But at the same time, not being aware of mortality would prevent us from trying to find a cure for it.