The manga's original ending wraps things up with this sort of cosmic justice—Kira's defeated, the world moves on, but it's all very public and final. Near and Mello corner him, and his death feels like a direct consequence of his own hubris. The alternate ending I read, which I think was a fan concept that got some traction, was wild because it hinged on Ryuk not writing his name. In that version, Light wins by outlasting everyone's suspicion through sheer, boring patience. He becomes this utterly hollow, paranoid god, ruling a silent world too terrified to even think a rebellious thought. Ryuk eventually gets bored anyway and just leaves, and Light's left with eternal life and absolutely nothing satisfying to do with it. The fate isn't death, it's an infinite, lonely victory. The main character's fate changes from a dramatic, narrative comeuppance to a psychological life sentence. That's arguably way darker.
I prefer the original, honestly. The alternate is a neat thought experiment about the emptiness of that kind of power without opposition, but it lacks the cathartic closure. Light getting outsmarted and dying in a panic on those warehouse stairs is just a more fitting end for his character arc.