The problem with human immortality would be human subjectivity, in the light of willpower and choice. Say humans could live forever, with the leaders of the human race, a group of violent and selfish tyrants, with unnatural subjective cravings, and the will and means to satisfy these urges. This immortal world would be messed up; hell, that never changes since the leaders could last forever, well consolidated.
With finite life, subjective human problems only last a few decades, with hope for a better world around the corner for the next humans. The immortal jelly fish, renews its mind, to avoid this.
If you look at people now, even with 80 years of life, many have a hard time finding joy in a basic life. Instead they need constant and stronger stimulation. If extrapolate such people to immortality, what happens if the stimulation tech peaks after 500 years, and now they are board but will live forever? Fear and death become the ultimate stimulants, to feel alive.
The atheist would have a much more difficult time with immortality, because they build their philosophy around the assumption of a short life. They don't put off things beyond this perception of the future, like immediate gratification. The religious people train their mind with the goal of immortality in mind, albeit, in another realm. This practice, even if imaginary, or not, might well make it possible in terms of the attitude needed to make immortality useful and practical.