At the end of the game it seems that Margras grabs a hold of the Idiot Ball and doesn't let go: Margras is screening Katana's run on the Prometheus Driver with his shields, but when they go down he refuses to let Katana swap positions with him despite the fact that shields can regenerate. He does this because after he realizes he was partly responsible for destroying an entire planet in a fashion similar to Terra's destruction of his own, he lost any will to live beyond his goal of destroying that horrendous weapon. Margras commits suicide.
The post-credits scene shows Katana and Ellen standing in a revitalized Acheron. The Terra Central Government destroyed that planet to enforce their power, and ADAN couldn't revive it because their leader focused purely on vengeance instead and led to the organization's demise. So who revived the planet? Terra Central. Katana was in command of the TCAF fleet that saved the Earth from being terraformed, no pun intended, by ADAN's superweapon. He likely wanted to honor his friend's sacrifice/suicide by restoring the planet that Margras fought for, and the TCG, both grateful to his team and realizing how close they came to their planet's certain doom at the hands of their own apocalyptic tactics, at the very least complied with his request.