Well, it is. But it's not.
Your value to your organization is not what you do, or, to be more precise, it’s all about what you can get done smartly. Joel Splosky makes excellent points in his Joel On Software blog about the two important prospects for a candidate being “smart and gets things done.”
In that spirit, your ultimate value to your organization isn’t the very specific role you serve today, though you can’t neglect the need to excel in it. It’s what you bring to the organization in the long term. If your real asset is your detailed knowledge of Windows NT 4.0 or some arcane details about Sendmail routing, you’ve likely reached your pinnacle.
That’s fine if that’s your goal. No, really. If that’s what you want, pursue it. Senior professionals, though, are force multipliers. They add things beyond a simple set of skills. Don’t neglect that area of your own development.