Without commenting on Seagal's rank in particular, there are considerations other than skill that go into a seventh dan promotion and weigh as heavily, or more so. You also have to consider that the hombu, i.e. Doshu, doesn't really know most of the people that get promoted.
I completely agree with you and yes, other factors certainly weigh in heavily. I don't pretend to know the working of Aikido politics at those altitudes. However, he must have some reasonable trust in those who recommend that someone (in this case Seagal sensei) be promoted-Isoyama sensei, Abe sensei, etc. I do find it curious when someone earlier said that Seagal sensei has been promoted to 8th dan (if true) when others such a Saotome sensei, Yamada sensei, etc have not been promoted first.