[ufmace]

It's a little dicey. Consider also that the developer who can't be fired also can't be promoted. Do you want to be the coder who made something so essential yet incomprehensible that nobody dares to fire you or promote you? Or do you want to be the coder who built something that's awesome and essential, but also simple enough to understand that anybody can maintain it, so you're free to move on to better opportunities at will? There's always somebody willing to employ the coder who can do the latter.

Then again, there are always people who have so much specialized domain knowledge and experience that it just can't be represented with any amount of good architecture, comments, and documentation. If you're in that position, do the best you can to make things clear, and try to be reasonably helpful in finding and training a replacement when the time comes.