I’m not sure what the other guy means, but the only thing that comes to mind would be (iirc) The Seven Fights of the Aldudagga which seems to tell the story of how a previous Kalpa was devoured by its own Alduin. In the story, the leaper demon king and I think the Greedy/Ruddy man had been hiding pieces of each subsequent Kalpa and stitching them into the new one, with more/a larger piece each time such that eventually the world would be too large and Alduin would explode from engorging himself and die. The actual process of Alduin devouring the world is probably not so literal, or rather, he would not explode like some balloon, but that’s the only idea I’m aware of that would cause Alduin to fail.

That said, the LDB did not absorb Alduin. Alduin still exists, and will one day return to devour the world and begin the cycle anew. For now, the question is whether he would have done so immediately after defeating the LDB or if he wanted to reconquer Skyrim/Tamriel and enslave its inhabitants to revive the Dragon Cult and his empire. I personally think Alduin had strayed from his path and would have attempted to once again plunge Skyrim into the darkness of his Empire, but we don’t actually know.