There is quite a bit of "mis-information" floating around on this post. There is an element of truth regarding Comm/Elint and other electronic equipment getting smaller and more reliable. However, this has let to more equipment being added to the mission requirements. So, it becomes a trade-off, while less volume and weight are required for a piece of equipment more equipment is being installed.
My guess is if Boeing were approached for an E-4B replacement, it would get done. The problem with some of the aircraft suggested is that after EMP hardening, the payload would be terrible. The AF-1 and E4B are about 100,000 pounds heavier than their civilian counterparts due to the additional equipment, EMP, self-defense, two APUs and other modifications. At the end of the day, it really doesn't make much difference if it's an F or 8I for the airframe. The freighter would require serious modifications to the interior, remember the freighter only has one Lav and it's in the upper deck, it also has the smaller hump which results in less interior volume than the 8I. Plugging the window if an 8I is used is a very minor task in the big picture of things. A/R has already been added on the E-4B and I would be willing to bet, the replacement would have the same capability added.
Just my opinion on the thread.
I fully agree
Regarding the weight (over 100.000 lbs) of all the equipment :
747-8F aircraft are structurally built with an an increased MZFW of 727.000 lbs (329,762 kgs), incl. increased strength floor beams, allowing for heavier locally placed loads.
747-8I aircraft are structually built with a lower MZFW of only 651.000 lbs (295.289 kgs) and reduced strength floor beams for weight reduction.
So, it's more easy to adapt a 747-8F, with some interior modifications, than to increase the structural strength of secondhand 747-8I aircraft.
Note : with a civil 747 Passenger - Freighter modification only part of the structure can be modified for the higher operating weights. The not modified passenger structure will receive increased wear and tear (quote of Boeing regarding converted freighters ).
Operating a twin over the ocean, you're always one engine failure from a total emergency.