Your functions won't cease to exist. All of the code in your compiled and linked code image gets loaded into code-memory (Flash) and it stays there throughout the run, through resets, and through power-cycles, until another image is loaded over it.
Are you confusing pointers to functions with pointers to a function's local variables which occupy RAM memory, specifically within the function's stack-frame which is transient, being created and "destroyed" as the function gets called and returns - and successive stack-frames won't necessarily be created in the same place.