Hmm, if Kahurangi retires in 2011, that's only 2 years as a vice admiral for Sulu, and in my opinion, that's too short to be considered eligible for the top dog position.
When it gets to the very top job my understanding is the candidate is picked directly by the Federation Council, which basically means the thread will get a vote from among the vice admirals. Possibly we could force the selection of a Rear Admiral or anyone we wanted via write-in, if we could explain to the QM a logical reason why the Federation Council would decide to ignore Starfleet ranking structure and pull in someone from the lower ranks or outside the system. (Such as a situation where they really hated the Admiral and all the Vice Admirals were seen as tainted by association or something like that.) Choosing not to consider 'time in rank' seems entirely in character for the Federation Council, though.