Long ago, when I was single, one of my managers refused to give me new development work because I was dating a guy who lived out of the state. “You'll go marry him and then where will I be?” he asked. “I'll be down a developer.”
“If you don't give me good work to do, you'll be down a developer,” I warned him. “I'll be looking for another job.”
“You wouldn't do that to me,” he said, shocked.
“Sure I would. Why wouldn't I? I'm not married to this guy. We are just dating,” I replied.
I didn't marry that other guy. I married Mark, who was—and still is—local. I did look for a new job, because I wasn't working on the interesting work. I was working on the old boring maintenance stuff, not the new exciting work.
You see hiring managers making decisions like this all the time. Do they try to hold testers in test jobs, especially now that agile testers have more automation capability and could become developers more easily? Or do they try to keep technical support staff in tech support when they could move into development—or the other way around?
It's a mind reading problem. If you are a hiring manager, don't assume you can read the mind of a candidate. You can ask behavior-description questions. You can ask elimination questions. You can even ask for a commitment time, “I need you to commit to this position for x months.” But, if you as a company can't commit to a position for that length of time, it's amoral to ask that of a candidate.
Especially when you hire from within, take the time to perform a job analysis on the open position. Do write a job description. Develop your behavior-description questions. Know what your elimination factors are. You might surprise yourself. What eliminates one candidate from the position might not be what you are worried about at all.
My old boss? He did replace me, but as he said, the humor quotient decreased in the department. And, so did the maintenance. The customers noticed and were unhappy. They had to hire a couple of people to replace me. I had taken years of solution-space domain expertise with me. It's difficult to replace that with one person. With two people, they could pair and learn from each other.
Ask, don't assume, you know what's going on in a candidate's mind. It's too expensive to assume you know what the other person thinks. Just because you work with that person daily does not mean you have clairvoyance. Let's leave the mind-reading to the professionals, ok?