As much as he wanted to be with Arianne all the time, Magnus was still aware if he didn't have enough preparations before the apocalypse, his little bird would not live well. He wanted her to have the best, a life without worries, which he wasn't able to provide in his past lives.
If only the space she had given him allowed the owner to bring another person inside, Magnus would have built a house along with the lotus pond she deeply liked from their childhood. Unfortunately, everything doesn't go on one's way.