In 1939, International Business Machines Corp. was removed from the average and replaced by AT&T. That was a fateful move. Over the next 40 years, before IBM was restored to the Dow in 1979, AT&T, largely a regulated utility, saw its share price increase about threefold. IBM, on the other hand, greatly expanded during the war and then moved into the soaring computer industry. Its stock increased about 22,000 percent between 1939 and 1979, one of the great investment success stories in Wall Street history.
Had Dow Jones just left well enough alone and stuck with IBM, the whole history of the average — and thus the perceived history of Wall Street in the postwar years — would have been different. The Dow would have recovered to its 1929 peak years sooner than it did, and marked many other milestones, such as reaching 1,000 points, earlier as well.