"Unfilled" command spots on bases don't go empty (and nor as you're intimating, potentially up for grabs by a DEC) with regard to the whole system, they are taken up by a commander in HKG. Required additional command positions are filled from the hub, and are not required to be filled directly at the spoke.
And....
Gaining a command on a base requires normal command seniority as well as gaining that command spot through an open PBPA base bid.
Sure, a B747 command might go more junior than the normal command seniority due to the undesirability of the roster, but they are required to be offered in seniority order amongst all officers on the seniority list. We need to get away from the separate freighter fleet mentality and stop the rot!