Eventually there will be a need to come up with a replacement for the C-17 - and yes it will be expensive. But it'll likely be a far better airplane due to the technological advances in the meantime.
I know I'm stepping into a discussion I'm not really qualified to be in but, I can't help but think of the C-130 (J) and how it has been transformed with upgrades whilst still maintaining the reliable work horse platform. The line is closed so it's a moot point, but there is precedent.
(Now, if only someone could tell me why the B52 was never re-engined...)
