Death Note Theory

Misa in the series “Death note” serves as a form of comic relief in the series. She is a woman with a death note aside from Light Yagami(yes light is his name). Misa was saved by a Shinigami named Jealous. By saving her life he gave the rest of his life force to Misa. So in total Misa would live however long Jelous would live plus Misa’s life. Intuitively Misa should have been immortal because she had her life force extended. However we don't know this definitively in the series; she dies in her early 20s. For this post I will be breaking down(no pun intended) the age Misa should have lived. The Shinigami are bound to follow a person with a death note. They typically offer a deal with the user. They offer Shinigami eyes(It helps the user with the notebook) for half their life force. Effectively if the person is destined to live to 20 years then that person would die at 10. However the user can refuse the offer(as Light did) however Misa did accept the deal twice in the series. Shinigami can die if they save the life of a human. Subsequently if they do the rest of their life would go to the person. In this series Misa is saved by 2 Shinigami Rem and Jealous both of which die in the series. So to recap, Misa's life was extended twice by two Shinigami and cut in half twice (so effectively she would be left with a fourth of her original life force). So if Misa died in her early 20s then 20 * 4 = 80 she should have lived to her 80s roughly. So one question has been boggling my mind about this. If she had 2 supernatural beings increase her life then why would she live to just eighty? Keep in mind that the Shinigami in the first episode just kill to gain more life force to live endlessly thus accumulating a longer life. So why would it be low(not that 80 is a short life that’s long for a human). I have a few theories for why this happened. My first theory is that both Rem and Jealous had to have a short life span already. The Shinigami realm had a lot of sad demons (shown in the first episode many sad demons along with Ryuk and Jealous). Maybe Rem and Jealous both stopped writing names in the death note to let their life force dwindle due to an inability to handle sadness. The Shinigami world is a very depressing land. Ryuk even highlights this a few times. Once in the beginning of the series he shows that he thinks there is no hope the Shinigami land won't get better and the second when Ryuk brings back food from the Shinigami land. He brought back a small old, worn, degraded pepper Misa eats it and says “uh it tastes like sand”. Generally if someone lives in a world then they adopt the feelings of the land around them. My second theory is that Ryuk stole more of her life. I blame Ryuk and not Rem because Rem literally gave her life to save Misa. Mathematically Misa’s life should have been cut in half twice leaving a forth of her life left as stated before. So mathematically this means that Ryuk had to take more than half of her life when giving her the Shinigami eyes again. In both instances Shinigami seem to be lifeless beings or deceptive liers to their users.