My girlfriend is a nine-tailed fox. Why didn't she die in the end?

In 50 days, the elixir was returned to the tail fox veterinarian, saying that Daxiong's life span was cut by half, and how long he lived depends on how long he lived. Moreover, Nobita said that Grandpa can live to 120 years old every time his life expectancy is halved, so he can live to 60 years old. See 13 for details.