Where does this line get its equivalent of the Mandate of Heaven? Some claim to that was generally necessary in eastern faux-autocracies if you didn't want things to immediately degenerate into an orgy of shanked kidneys and burning cities. It was enough that in Japan kept a dynasty going for 125 monarchs despite the fact that the position of Tenno was almost completely without power for most of it. Japan needed the figurehead.
I'm assuming its a holdover from the time of Indra and Asura, Hagoromo or even the Land of Ancestors. Even a minor recognition by one of the beings treated as living gods would mean something major.