according to Flight this week the USAF are already looking at a K-46 successor for 2030-ish.......
The KC-135 has flown for over 60 years and will continue to do so for another decade or two. Maybe longer. The C-17 is planned to fly for another 100 years. It's likely that the "successor" to the KC-46 will be a derivative of the KC-46. But there is one big fudge factor in any future large aircraft programs. Boeing is on contract by NASA to further develop the Blending Wing Body and Low Drag Trussed-Braced Wing concepts. If there's a breakthrough in those technologies, USAF may ride on NASA's coattails to make an operational version.