Back when Bill Berry retired from R.E.M. in 1997 most people were, like, "Big deal. He's a drummer... he can be replaced." And besides, Berry had suffered a brain aneurysm a couple years earlier, so health was an issue, too.
But then band went on to release the albums Up, Reveal, Around the Sun, Accelerate and Collapse Into Now without him, and proved that no, some drummers can't be replaced.