I honestly do not understand why these MCs just shut up about the truth instead actually saying it and asking the people in their reincarnated family for help in assessing and dealing with the situation. Dude chased after a Demon Lord out to destroy the entire human race in a desperate life-and-death battle. And after reincarnating, he finds out "OMG human race's men can no longer use magic and all magic that can be used has fallen to so low a degree!" and doesn't even hold a sense of urgency at all. Like bruh, it's clear that it's highly possible the Demon Lord that got there before you did something in the past before you were born again, and caused the entire human race to get weaker in combat as a result. Don't you think you should be telling your reincarnation parents the truth that you're a frkin' Great Sage and maybe get your dad to actually see if he can actually still use magic in order to find out if society is just being manipulated with disinformation?