In my experience the opposite is true. My personal goal on any project is to make myself replaceable. There's nothing I find more tedious than having to work on the same thing for years because no one else can take over from me.
So you admit that you aren't replaceable? If the company mandated that you should always be replaceable then you wouldn't need that goal, it would always be fulfilled. And working in a way that makes you always replaceable isn't fun.
Edit: What we learned from your comment is that making yourself replaceable is fun, but being replaceable isn't fun, since as you say you work to become replaceable so you can start working on something new rather than to stay replaceable forever.