His logic is dumb. Reincarnation is dumb. Isekai is dumb. Just say it’s you and be done with it. Want to pursue magic? Your disciple has 400 years worth. Want to explore the world? Your disciple has 400 years worth. Want to live a simple life? Your disciple is basically immortal cause she looks like 20 but lived 400 years worth, she’ll stay with you your entire life. Did I mention that you reincarnated 400 years into the future? It’s so dumb. I’ve yet to find a good reincarnation manga that has actual plot in it and has isekai like themes. Like, where’s the villain? The hero? The whole point of the story? To pursue magic? To live peacefully? Become king? Literally everything would be so much easier if he just told her, “I’m yo teacher, now show me the 400 years worth of magic”.