I suspect that in such universes that are not destroyed very quickly, an early user creates fail-safe spell constructs that limit such destruction by future users (including themselves under most conditions). This does leave open the possibility that some primordial magic user with root access still exists somewhere, and is very careful to use such power only when absolutely necessary, and only with the minimum weakening of ordinary constraints.
I suspect that in such universes that are not destroyed very quickly, an early user creates fail-safe spell constructs that limit such destruction by future users (including themselves under most conditions). This does leave open the possibility that some primordial magic user with root access still exists somewhere, and is very careful to use such power only when absolutely necessary, and only with the minimum weakening of ordinary constraints.