They can replace you if you are just a coder. If, on the other hand, you work on an important product, acquire domain-specific knowledge, and know the code inside-out, then replacing you with someone else will be a lot harder.

Think, for example, about the folks who work on the Microsof's C/C++ compiler. I doubt any of them need to worry about their job. On the other hand, if all you did is implemented the Shutdown button for the next release of Windows then, sure, you can be easily replaced.