Yusuf Zuck's achievements today all relied on being Invisible, so he was acutely aware of the significance of invisibility to him. One could say that being Invisible could avoid death and also allow one to do things beyond others' imaginations. Hence, he did not want to lose this Skill, nor would he.
Regardless of whether the Higgins King was willing, the fact was that Yusuf's dependence on him was mostly for utilitarian reasons. All he wanted now was a place to settle down and ensure the safety of his two ladies. He could not possibly follow the Higgins King for the rest of his life.
Currently, the Higgins King was still a Prince, so not kneeling to him and occasionally joking with him was acceptable. But once the Higgins King truly became the Emperor, Yusuf would have to kneel upon seeing him, and his whole family would have to kneel; joking would be out of the question.