Once upon a time, America's carriers represented our sole long-range offensive punch. They were the ultimate platform in platform-centric naval warfare. Submarines, even nuclear-powered subs, still had a comparatively short attack range. Our surface vessels had guns with short ranges and at best primitive missiles. Their most important role was to protect the carriers.
With Harpoon we gained a missile that could be--and was--put on all our surface ships and submarines. We reduced the critical importance of our best platform--the carrier--and distributed our offensive power throughout the fleet (and in the air with air-launched missiles). Our enemies could no longer hope to cripple our offensive power with a dozen large missiles striking our carriers. The Soviets trailed our carriers during the Cold War so they could try to decapitate us. But this is no longer possible since our carriers aren't the sole source of offensive power.
As the Navy works on network-centric warfare, the ability to mass effect both offensively and defensively from widely scattered platforms, the importance of individual platforms is decreased even more. This reduces our vulnerability to the loss of individual platforms. Other assets can fill in the hole seamlessly and our Navy's targets will never know that a missile from a different platform destroyed it. It will be an irrelevant detail.
Our carriers will continue to be valuable in niche markets where enemies cannot challenge our Navy, but against powers with weapons able to strike our carriers, it will be folly to send the big ships out to die.
Strategypage writes (as I have) about the possibilities of Chinese land-based ballistic missiles targeting our carriers:
China is developing the technologies, and it's only a matter of time, and willingness to devote several billion dollars to the project, before they can actually do it, or at least try to. If the Chinese ASBM works, naval warfare will be changed forever.
The Ford class carrier may be the ultimate in carrier technology, but that means nothing when platforms take a back seat to networks of smaller vessels that can swarm the platforms. The Ford will simply be the most expensive and advanced target ever built.
