This statement amounts to so much hot air and speculation from a perspective that doesn't understand how business, let alone the radio business, works. Let's break it down:
"In the case of this dj (who left somewhere else), the 6+ numbers weren't good on the stations she was on."
As has been stated before; no radio station sets their performance matrices on 6+ numbers. Assuming this person was hosting a show on an Alt-formated station, then the target demographics would likely be 18-34MF. Unless you have access to those numbers, your assumption isn't worth the effort to type it.
"It could be a case that management liked her, but perhaps people would rather listen to someone else." Assuming she left on her own as you stated; what would management's feelings for her matter? Answer: It wouldn't would it? Oh, and management doesn't keep talent around because they "like" them. It's a business. If the talent performs and tests well within the timeslot and target demo, they typically are allowed to continue collecting a paycheck.
"She wasn't "objectively" a bad dj, but very self-absorbed and annoying (though I'm sure some forum members think the same about me.)"
Your personal anecdotal opinion, not based on actual performance of the station or individual.