Someone could check me on this, but I think it wasn’t about her literally becoming a queen. I believe Kurtz said something about her becoming the leader of whatever is left of her people. I suppose that could be interpreted as her becoming Queen to any survivors of Alderaan, but I always interpreted it as her becoming the leader of whatever is left of the Alliance at the end of the movie. Since she is Princess Leia, I figured people misinterpreted that as her becoming Queen.

EDIT: Also, I do see what people are saying about Han dying might not working, especially with his survival being ambiguous at the end of ESB. Like, we would get him back just for him to “die” again. I think it could have worked if Han was given a clear choice between surviving in order to live a long and happy life with Leia, or sacrificing himself for a greater purpose than his own happiness. If Han just died in battle, without having the choice between living or dying, it probably wouldn’t have been as satisfying because it wouldn’t have been character-motivated at all.