We already have a updated C-130J with new quiet engines with better performance and glass cockpit. and C-17’s that haul more cargo why buy Airbust when we can buy AMERICAN.
Because barring any further foreign orders in the next six months, the C-17 line is due to shut down permanently in mid 2015.
After that, if the USAF wants to buy more transports with larger capacity than the C-130J, it's either the A400M or a decade long new development program.