I feel like that's a bit unfair and superficial, like arguing any movie with a romance is unoriginal because romances have been done to death since the dawn of film or art really. It handles the conceit of a multiverse in a decently novel way that advances the plot and plays to the film's emotional stakes. I think that's original enough, especially in the context of modern film. Not to mention comparing Avengers to this film is effectively comparing a superhero epic to a family drama diving into trauma and love. It's an entirely different premise and genre in my mind.