Even if your work, knowledge, and contributions make you irreplaceable in your organization, your employer is never going to admit it. For one thing, they probably aren't even going to be aware of it until long after you're gone. They probably think that there are dozens of people out there who can do exactly what you do. For another, if they did admit how valuable you were, they know that you'd likely want to be paid fairly for that fact—and that's just never going to happen. No, it's better t…