[foxyv]

If you cannot be replaced, you are un-promotable. Your entire career becomes tied to a single point of failure. Then, when they replace that piece of software with a new product you have no experience elsewhere.

[jerlam]

Implied is that you work in a company that is growing, that will promote you.

If you work for a company in stasis or worse, shrinking, then being replaceable means little.

[francisofascii]

So maybe "interchangeable" is a better goal. They can replace you, but keep you, just somewhere else.