Am I wrong about job security as a sub?

First of all, I know that you can be thrown under the bus at any time by a school or even the entire school district. But bar that, I am led to believe subbing is a very secure job. The work is steady in my district. I know there is no ageism as you just show up and there are tons of retired teachers. I just don't see any downside to working this forever if I wanted to and can't afford to retire.