Well, that's how I would logically approach the situation. If someone is busy recovering from pneumonia, and I needed to run a bank heist, I'd pick up their cousin or someone else available.
Yeah. If you regularly force your players into combat without an advantage, or even sizeable disadvantages, the combat has to be pretty non-deadly for it to not result in death. Lethal games should make combat rare and dramatic.
Mark: Theoretically, if you wanted to do a chosen one game you could make all the player character high-powered to give that epic hero feel, while the deadliness of the system would still make them feel at risk. Then if one character dies, that player could play as a weaker underling. Every so often the players would meet another epic hero of destiny at sort of check-points. That could be interesting.