But how often did we actually see admirals doing that in the TOS era? I recall Commodores having expanded responsibilities, but admirals were basically stuck behind a desk somewhere, and never on a ship. DS9 is over 80 years in the future from TMP, and in the intervening years, the Commodore rank disappeared and many other changes were made.
Yes, but the Star Trek
universe was a great deal smaller in TOS than in DS9. It's entirely possible we would have seen that sort of thing had there been more than three seasons of material for the writers to work with. As it happened, the TOS films themselves did a lot to expand the ST universe beyond the scope in which we'd originally seen it. With Kirk commanding Enterprise
as an admiral, it opened the door for a lot of things to happen that hadn't been shown previously.