Deep within the text of Nicholas Pileggi's book, Wiseguy, is a hint, a single clue.

Henry Hill recounted that at the taxi stand he hung around when he was a lad were some old Murder Inc guys.

That right there confirms any possible suspicion that all of the Murder Inc crew members were not in fact caught. Some got away, and they are not in the history books.

edit: Henry Hill was born in 43. The hierarchy of Murder Inc was supposedly executed around 42. So if Henry saw some of these hitmen from the original Murder Inc at the taxi stand when he was a youngster, it would imply that these hitmen outlived the prosecution and destruction of Murder Inc.