Would there even be a need for forgiveness to exist if people never reacted with toxic or unhealthy anger?
Forgiving others is not done exclusively because it has excellent psychological benefits, shown by research. Forgiving others also is good in and of itself because it is a moral virtue (as are justice and kindness and respect). Showing goodness as the goal of forgiving (rather than deriving a psychological benefit) is sufficient for forgiveness to be a part of your and others’ life. To address your point directly, as we both know, reacting to injustices only with temperate, short-term (not unhealthy) anger is not likely as part of the human condition. Thus, the need for forgiveness, for psychological reasons, will continue to be alive and well on this earth.