Fair enough,but the airframes are from the mid 1980s,and they first flew in the late 80s. The issue is the E4B aircraft usually are deployed at the same time,ie,one E4B is deployed everytime the VC25 is deployed. The problem is the E4B aircraft are even older,ie,from the 1970s.
Both the VC25s and the four E4B aircraft are based on the 747-200 from the 1970s and are considered first generation 747s(the 747 is in its third generation),and almost all 747-100,747-200 and 747-300 aircraft have been retired from airliner service with the last few 747-200 aircraft serving as freighters. The IRIAF is one of the last military operators of the 747-100 and 747-200 interestingly enough.
The E4B aircraft have been put forward for retirement,they do have a reasonable amount of airframe life available,but it makes me wonder how easy parts will be to get as time progresses.
A new aircraft especially based on the larger and more advanced 747-8 will not only have greater range(meaning less refueling stops and less need for tankers to accompany the VC25),but it will allow for greater functionality to be added to the plane too. The order is meant to be for three replacement planes for the VC25s,so I get the impression,they are also partially replacing the E4B too in some ways,ie,one VC25 replacement in the air,one on standby and one in maintenance. Currently out of the four E4B aircraft only one is actually on active duty at any one time.
If(and it is a big If),they are intending to have the VC25 replacements take over more of the functionality of the E4B,then in some ways,it will mean the USAF can gradually retire them from service,then directly replace them which would certainly lead to cost reductions.
Sure you can upgrade old aircraft but the MRA4 scandal showed us that sometimes buying new ones is actually more cost effective.