Out of interest, what does the US have planned to replace the C17s?
Right now, absolutely nothing. Current USAF plans are to fly the C-17 for 100 hundred years. That's right, a full century. Not too outrageous an assumption given that the
youngest B-52 is already over 60 years old and they will fly for at least another two decades. And whereas the B-52 was never designed for such a long life, the C-17 (especially the 40th and later ones) have a 30,000+ design service life and USAF is already working to ensure service life to three lifetimes, or over 90,000 hours. In other words, your great great grandchildren could be flying C-17s in the next century. So USAF is in no hurry to develop a replacement. Who knows, maybe they'll have transporter beams by then and there'll be no need for air transports. ;-)
And interestingly, while RAF leased C-17s as in
interim mesure until enough A400s were acquired, that has changed. Until recently RAF C-17s were "offically" strictly strategic transports with no tactical mission. But RAF has used them tactically many times and is now making that official. All RAF C-17s have been upgraded to the latest block 19 standard (not even USAF has that.) and RAF will be using all of the C-17's capabilities for the first time. This is a big deal because the UK's airdrop equipment is not compatible with C-17, so a lot of new airdrop equipment will need to be purchased to accomplish that. And RAF C-17 crews will need to be trained not only for these tactical missions, but also boom refueling, which was (as I undersand it) not part of the original lease agreement. I don't know what this means for A400 which was supposed to be the premier tactical airlifter.