This is one of my favorite isekai, but I think you got things reversed. Souma is the realist and therefore works with what is logically the best for his people. Maria is the idealist and works what she FEELS is right. The Humanity Declaration is what she BELIEVES will unite the other kingdoms while Souma already sees the pitfall of that.
Humanity often comes up with ideas based on emotion rather than logic. Though it may SOUND good on paper, it rarely works in practice. Prohibition, the Helsinki Accords, defunding the police, etc. You have to look at it from all sides. And sometimes you have to make the hard decisions, just as Souma must do at times.