That's one possibility, but I'm skeptical. If that were the story, then the movie wouldn't really work as its own self-contained movie, only as a prologue to later stories. That would be bad storytelling structure. A lot of the biggest franchise-movie flops have failed because their makers got so fixated on setting up future movies that they forgot to make the current movie satisfactory and thus produced bombs that never got the sequels they were setting up (e.g. The Amazing Spider-Man 2 or Tom Cruise's The Mummy). Ideally, whatever the story of a movie is about needs to be resolved within the movie, even if it leaves a lingering thread that sets up a future movie. For instance, Quantumania introduced the Kang threat to the movies, but it gave a conclusive ending to the conflict with its specific Kang, leaving the rest as a teaser.
However, I suppose one possibility might be that their world's destruction is the end of the first act, while the second act is their arrival on Earth-616 and the third act is them saving 616 from Galactus like they failed to save their own Earth. That way it's a complete story, not just a fragment of one. And it's a debut that lets them prove themselves as effective heroes rather than failures.
Still, I'd prefer something less pessimistic, something where they succeed in saving their Earth, learn of the multiverse from Galactus, and choose to explore it as their next adventure.
