Anyone can already take your job, as long as they are willing to relocate, or your company opens a new office near them.
It is more important than ever to differentiate yourself. To do that you need to find something to do that you love, and then find (or more often make) a niche in that that that you become one of the top ten people in within your context.
By context I mean you don't need to be the best in the world, you can be the best at X within the Y industry, or within defense consulting, or in the U.S. And you don't need to be the best (though see the quote about being the only), but at least in the top 10.
There are two quotes I am fond of that say similar things much better than I can:
"Your work is going to fill a large part of your life, and the only way to be truly satisfied is to do what you believe is great work. And the only way to do great work is to love what you do. If you haven’t found it yet, keep looking. Don’t settle. As with all matters of the heart, you’ll know when you find it." - Steve Jobs
"You do not merely want to be considered just the best of the best. You want to be considered the only one who does what you do." - Jerry Garcia