From what I have seen over the years of watching the boards is that people buy Chromis for their shoaling quality and then slowly watch them disappear until there is just one or two. I think they are aggressive toward each other and slowly pick off the weakest ones.
There are a pair of clowns at Gallery of Pets that are over 25 years old. SW fish will live a long time if they are kept properly. That is why it irritates me so much when I see people put fish in a tank that is too small, or too many fish in a tank. After 6 moths they say everything is happy. 6 months is barely a beginning. Too many of us look at things too short term in keeping our tanks.