I had a theory about this manga that I wished had come true: The princess wasn't really that nice of a person initially, but the experience of losing her family to a rapacious warmonger made her dislike violence, and that the MC aborting that tragedy from the timeline would inadvertently cause her to not be the woman he fell in love with anymore. After that the story could go two ways: Good Ending, where the MC has to really work to get her to that place of being a good person, or Bad Ending, where she ends up being a bad guy and the MC has to kill her off in a butterfly effect type scenario.
But I guess we're just doing the typical shonenshit thing where everything the MC does works out just fine with barely any nuance.