Mareo187

Time travel no dude, first of all the original creator and Harada planned for the Mishimas to be the centered piece and central protagonists of the Tekken world, meaning no Mishimas means no Tekken overall. Also Kazuya since was the original protagonist, and his son and father both gotten a redemption story. Making Kazuya evil again wouldn't make sense since Jun is back physically.

Kazuya is the only Mishima who still didn't receive a revelation yet and also he has bigger issues that focus not just Heihachi's resurrection Which is Connected of what the Mishimas truly are and Jinpaichi's Resurrection, but he also have the Hachijos his mother's maternal side lurking in the shadows waiting for them to be eliminated. Kazuya is the center piece character that connects to all conflict unlike destined one son.