Generally speaking sensei is often as much an honorific as it is a designation.
I addressed my daughters kindergarten teacher as sensei. I also refer to my dentist, my budo teachers, my boss and the old man down the street, as sensei. In the latter case it has a lot to do with the original meaning which is "before life" or more eloquently "one who has gone before".
In Japan, think of -san versus sensei as mister versus sir and you wouldn't be that far off.
In the dojo there are several people I refer to as sensei. All are above san dan but that is often not enough. Age has a lot to do with it even though there are some that are younger than I. I think the main thing is a long term commitment to teaching Aikido even if they are not the dojo-cho. For instance, I can not think of one ex-deshi where I don't use that honorific since one of their main functions as deshi (there is usually only one with a tenure of 2-3 years) is teaching classes. When you come right down to it - sometimes its just a matter of feeling.
I don't think the term has anything to do with establishing authority. The thing is everyone knows where they stand in the order of things - titles are superfluos.
That last bit does not mean they shouldn't be used but the application, or not, is not really so rigid.