I think in many cases people over-estimate their worth. Where I work I am the only real developer. I wrote and maintain all the products.
This makes me pretty indispensable. It also helps that before I arrived the guy in charge of the company had a bad time with at least 3 developers. If there is something I cannot do he tasks me to find someone and manage them so you know...
Obviously, guy could find someone else. Someone else could take the reigns of what I am doing. However transition time and the guys past experience with developers makes this very unlikely.
While I know nothing about you and your skills, this doesn't have to be always that simple. People who are "the only developer" in a company often get detached from the developer community, living in a kind of technology bubble they created. These people always solve the problems "their way", often obsolete, using inferior tools. They can write tons of code, when there's already a tool which does the trick just a google away. Again, I am not talking with you personally in mind, but simply being the sole programmer doesn't make one indispensable.
Please do not believe he is unaware of this situation and is not made nervous by it. Unless he is a fool - which he cannot be so much because he trusts you to get the job done :-)
He operates in a market for lemons, but one day it may be more comfortable for him to risk getting a Lemon Consulting firm than live with the stress of one guy who might leave and take the whole thing down with him.
I think, as a general rule, people overestimate the value of their own skillsets/contributions and underestimate the skillsets/contributions of others. It is probably a Jedi mind trick to make ourselves feel more important/smarter/what have you. Luckily the free market for labor mostly exists (ignoring possibly shady deals between certain large tech companies) and people feeling undervalued can test that theory by shopping their skillset around. I think more people feel undervalued/underappreciated than pursue the "shopping around" angle. There are lots of reasons/excuses as to why, but protecting one's ego probably has a role, acknowledged or not.
Once I was the only "indispensable" developer too. Also under a bad managing owner. When I left I offered to help in time of a crisis. Never heard of him again.
Ask yourself this: do you get paid for the risk of you leaving? And ask yourself this: can your family enjoy you when you feel bad?
I think in many cases people over-estimate their worth. Where I work I am the only real developer. I wrote and maintain all the products.
I learned this lesson the hard way, long ago, in exactly that same situation -- it took two times -- by attempting to use leverage I didn't actually have but believed I did.
I hope most people are less dense than I was (hopefully "was" is the right word there), because no matter what percentage of the work you do or how deep and unique your knowledge of a system is, you can be replaced. It may not be easy or quick, but if you make yourself a nuisance, get on someone's bad side, or any of myriad things in and out of your control, you can and will end up on the chopping block, wondering what just happened.
This makes me pretty indispensable. It also helps that before I arrived the guy in charge of the company had a bad time with at least 3 developers. If there is something I cannot do he tasks me to find someone and manage them so you know...
Obviously, guy could find someone else. Someone else could take the reigns of what I am doing. However transition time and the guys past experience with developers makes this very unlikely.