It's one thing to give preference to an equally qualified applicant/candidate, but it's
NOT OK to give preference to a less qualified or unqualified applicant/candidate. It is far better to leave a position vacant and let the next person in line be an acting thus-and-such, until a qualified person can be hired, than to hire an under or unqualified person.
If the person formerly in the position failed to train his or her next-in-line to be fully capable of stepping in without notice, and for any length of time,
shame on him or her, and that, in and of itself, is a dereliction of duty. If that next-in-line is incapable, even after training and retraining, he or she has to go. Inability to fill in for a supervisor, especially at that level, is an inability to fulfill the requirements of his or her next-in-line job/position. Taking care of continuity of operations (COO), especially in certain positions and levels is an absolute must.
Unfortunately, the Peter Principle. In some positions, it's catastrophic/deadly
Normally, it takes a minimum of 92 days to fire someone in the US Civil Service, and everything has to be done exactly right and at the exact right time (most of the time the supervisor messes up and it takes a lot longer)
I walked/taught/coached someone through doing this because one of his subordinate's incompetence and
refusal (not inability) to learn and perform the job to standard was going to kill people. As it was, that subordinate's less-than-four months' tenure in the job (including the 92 days we spent getting rid of the incompetent employee) cost thousands of man-hours fixing all the mistakes/on-purposes and preventing unnecessary death
Plus a LOT of money
I don't know if there are provisions in US Civil Service that allow for a faster firing in the event of something extraordinarily egregious, or anything pertinent in the Senior Executive Service (SES) level rules and regs.