I don’t have a problem with cousin marriages done now and again and when both partners agree to it but carrying it on generation after generation and not wanting/letting the kids marry outside the family obviously leads to problems..
Some people say because it’s been made halal there can never be any problem. Of course Islam permits it but that doesn’t mean we should carry it on every generation, meat has also been made halal but too much of it also leads to obesity, diabetes, high cholesterol etc., too much of anything tends to lead to problems..