In the real world, sure. In fiction, I disagree.
In the real world, all these characters have their own lives that continue with or without MC. But in fiction, they don't. In fiction, characters are only relevant to the story being told. The story of MC. Anyone who exists outside of that story, essentially, doesn't exist at all.
Which brings me to...
What happens to Sage and Chad if your MC doesn't get involved? Nothing. You, the player, have made the choice not to bring them into your story. As such, whatever path their theoretical lives may take is of no relevance to your MC. You have chosen to omit them from your story.
Making the Sage/Chad story continue without MC would, I agree, make his choices inconsequential. Making the story simply go away, unfinished, because MC has no reason to be invested and thus neither do we as the player, makes MC's actions of paramount importance, as it literally determines which characters get a place in the story at all. DPC doesn't need to write versions where characters' lives go when MC isn't involved, because it's MC's story. If MC isn't involved, they don't matter.