"They know they'll be leaving, whereas employees can count on being around."

I don't agree with that. A lot of employees don't expect to be around that long, because they know that they can get better raises by leaving than by staying.

Also, as the employee's institutional knowledge rises, that employee has a higher likelihood of getting shunted onto "critical" maintenance projects and support, which tends to make them dissatisfied with their work and therefore prefer to leave.

I've run into that one very often.