I'm puzzled by this, and the apparent 'rush' need. The current E-6 aircraft are not all that old - 1990 EIS - and the 707 airframe is known to be pretty robust (same structural technology as the KC-135 that's been flying far longer). The CFM engines are pretty much the same as on the KC-135 as well. I don't get what's so worn out that it needs urgent replacement.
Originally Posted by
Less Hair
Or they want to go back to the most classic, proven super robust steam technology available, for EMP resistance and such?
The 707 is a shinning example of the classic analog instrumentation - which by it's nature has far better EMP resistance than modern electronics. In contrast, the C-130J is basically a glass cockpit.