The original justification for the DD(X) was the installation of more cruise missiles (over a hundred) and one or two highly accurate 155mm guns that could support troops ashore. The gun have a range of over 150 kilometers and use high tech (GPS guided or cluster bomb) shells. The long range 155mm gun is another idea whose time has passed. Naval gunfire in support of troops ashore has not been seen much in the past few decades, and, again, the smart bombs are much preferred. This puts in question one of the reasons for the large size of the DD(X); providing enough space and energy for rail gun designs that are in the works. But the rail gun has the same problem as conventional guns; is there really a need for it. Although the rail gun can fire armor penetrating, guided, tungsten rods nearly 400 kilometers, there has not been much need for this sort of capability over the past half century.
The U.S. Navy's "next generation" destroyer program is in trouble. The current proposal, DD(X), is expected to cost nearly two billion dollars per ship, and not do much more than the current "Arleigh Burke" class destroyers. There are new technologies on the horizon that make it possible to skip the DD(X) and go for "next generation" ships. This can be done because the U.S. faces no threat on the high seas. With the end of the Cold War, the U.S. Navy possesses more than half the naval power on the planet, and most of the rest belongs to traditional allies of the United States.
Eventually it became obvious that more cruise missiles on ships was not a cost-effective proposition. Each missile costs at least half a million dollars, and it's much cheaper, and useful, to use GPS guided bombs dropped by carrier aircraft. Cruise missiles can't fly around for hours waiting for the ground troops to call for a smart bomb, and this sort of "bombs on demand" has become a key factor in the proven new style of ground combat. Current warships already carry thousands of cruise missiles, and there simply isn't a loud cry for still more.
Another innovations of the DD(X), a smaller crew (125 versus nearly 400 with current destroyers) is to be a standard feature of all new ships. This will be done by using the kinds of automation and low maintenance design common on commercial ships for decades.
Even the CG(X), to be built on the same hull as the DD(X), is in danger. The CG(X) is intended mainly to defend the carrier from submarines and aircraft. But there are new ideas for ships that can do that, including smaller carrier carrying helicopters and small, robotic, ships, aircraft and submarines that would be more effective in hunting enemy subs and defeating incoming aircraft and missiles. Thus the quiet (so far) battle within the navy over what future ships to build. There is no rush, and the gives the visionaries an opportunity to possibly kill the DD(X) and CG(X) and go for a truly revolutionary class of escort ships for the carriers.