Your average installed Mars probe widget is not a fungible component; it's been integration tested six ways to Sunday and is certified in its current configurat

Your average installed Mars probe widget is not a fungible component; it's been integration tested six ways to Sunday and is certified in its current configuration, as is the assembly of the whole thing, center of gravity, cleanroom status, the torque on every fastener and so on. Even if it's an off the shelf component, it may not be possible to replace it without a chain of sign-offs that would require months of work.