What makes someone irreplaceable?

When Todd replaced JP, there wasn't any backlash. When Ronny Woods joined the Stones, no real backlash. New Drum and Bass player for the Who hasn't really lead any one not to call it The Who. There haven't been that many lead guitarists that have been replaced. Ritchie Blackmore in Deep Purple is not a great example because they have had so many personnel changes.
If you replace the whole group like Axl did with GnR or Jeff Lynne did with ELO, there is some backlash. But as a whole, if you mess with the singers in any way, stand back. Are singers the only irreplaceable members of a band?
If you replace the whole group like Axl did with GnR or Jeff Lynne did with ELO, there is some backlash. But as a whole, if you mess with the singers in any way, stand back. Are singers the only irreplaceable members of a band?