Even though it seemed anti-climactic what the brothers fought for wasn't a lost cause, Earth didn't have another war after John freed them from Index's illusion and after a while. They started to rebuild, they found peace and solace in each other's arms.
And just like with their wars their peace also spread to the other planets, the practice of peace and love became commonplace. The hope that John had for the world blossomed and although nobody heard his message it was passed along by fate until it reached every single soul in the universe.
Do you regret it? Losing everything you loved just so you could save 74 billion lives who wouldn't even know what you did for them? These were John's fleeting thoughts, thoughts he pushed into the back of his brain because he could never fully get rid of them. Thoughts that ate him up from the inside out, thoughts that killed him even before he killed himself.
Hannah changed her name to Kayla Billoway, even though nobody understood why she didn't care. She didn't marry another person, she couldn't, she wouldn't soil what she shared with John even if it cost her very life. John would probably have told her to remarry, in retrospect that's what he probably would have wanted but this was her choice to make and she made the one she wanted to, regardless of whether it was right or wrong. "I know how much you hate yourself, that's why I'll continue loving you even when you aren't here anymore." Kayla pointed a knife at her neck and lodged it in. Dying next to the graveyards of everyone she ever cared about.