Many believe that this is the main message of the composition. In other words, the Egyptians could have gone straight to Jerusalem and destroyed the Temple then and there. The hiatus which could have provided a reprieve was bought through the agency of our mourning. Another important Tisha B'av lesson, our mourning and lament have a profound effect. We stay the Divine Hand, and, hopefully, inspire our souls.
Catharsis is a pedagogic device.
Destruction, debacle, disaster is a phenomenon beyond simple punishment. The worst sin one can commit against himself and Hashem is betraying one's role and place in Jewish destiny. Destruction is destiny denied. Esther heeded and salvation resulted. She intervened and victory was snatched from the jaws of defeat.