Ordinarily I'd say that not proceeding with the E-7 is a terrible idea. But given the advancement of Russian/Chinese missile range such that aircraft like the E-7 will be increasingly vulnerable to be sniped, investing in cheaper platforms like the E2-D as a stopgap measure until space-based assets are available might be the smart move. A single E-7 is roughly $1 billion, whereas a E-2D is at most $300 million.
What would the USAF rather have to deal with a particular emergency, a single Wedgetail or three Advanced Hawkeyes?