With short-term politicians, platforms remaining in service for decades and spending profiles stretching way out into the future, there is a very valid concern that we have no credible way to keep our aircraft manufacturing windows inline with military demand cycles.
There will be demand for many more C-17s over the next decade or two, but no economic way for commercial manufactures to meet it. High-rate and efficient production runs are great for civilian products, but not for the peaks, troughs and uncertain funding profiles of military procurement.
I understand where you're coming from, but taken to an extreme it would mean we'd still be building C-141s and flying around KC-135s - oh wait
The C-17 is already fairly old technology - the engines (the F117 is really just the military version of the PW2000) date to the early 1980s and the airframe the mid 1980s. Most commercial aircraft from that vintage have either gone out of production or gone through at least one major redesign in that time frame. Further, military aircraft don't get anywhere near the hours/cycles that commercial airlines do and last much longer.
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.