Reminds me of Death Note,
Except it’s not a game.

Halo, chief could’ve spent the rest of his life in a ship drifting through space with his hot computer gf

Mario

Seen the word “Bleakquel” used for it, but since it’s handier to point people at, we have TVTropes with Happy Ending Override. It’s so damn common because a happy ending means they cannot continue the story, so it must be ruined.