When Messiah establishes His Messianic Kingdom, the literal fulfillment of God’s promises to Israel will be complete and the mourning associated with Tisha B’Av will no longer be relevant.
The Jewish community does this the day after Tisha B’Av every year. They move on. They do not stay focused on tragedies. They spend Tisha B’Av recognizing the horrific events of the past, reflecting on the significance, and then wake-up the next morning to a new day.
“This I recall to my mind, therefore I have hope. Through the Lord’s mercies we are not consumed, because His compassions fail not. They are new every morning; great is Your faithfulness. ‘The Lord is my portion,’ says my soul, ‘Therefore I hope in Him!’” Lamentations 3:21-24