Lol, youknow better than this X. They are not interchangeable MC's. Players, at the start of the game, are given two options. To play as Aether, or to play as Lumine. Given that Lumine flipped Cult, we can assume that the narrative of this game follows the one that is spun when the player choses Aether.
In this case, Lumine is sent to the world of Teyvat years before Aether is, and completes her journey as a Traveler. At the end, she decides to join the Abyss, because her journey has jaded her to what we traditionally consider as "good". The player character, Aether, enters the world, and begins their journey as a Traveler. With his last memory being seperated from his sister, Lumine, your motivation is to go and search for her. At this point, you are not aware of the time discrepancy. In Chapter 1: Act IV, Aether finds Lumine, who reveals the whole time discrepancy thing. Lumine then outright states that Aether must complete the journey as a Traveler, just as she did once before, and then he will understand her choice. I will also add, both Aether and Lumine have a pre-established history of world-hopping. The story then heavily suggests that Aether, instead, will be searching for a different, better answer than his sister found, with his companion, Paimon (who shares the same trait as all the Archons in the game in that she shares a name listed in the Ars Goetia, and thus implying she is at least a fraction of a more powerful being), being a large reason for this.
So are you going to keep giving me generic, irrelevant theories that aren't relateable to this context, or are you going to find a way to properly substantiate this "gut feeling" of yours? Because I can debate the flavour all day, up to what I have played at least (approximately 20% through the newest content released 6 weeks ago).