Old planes do get reworks, as most recently seen in the A330-300, as well as numerous other aircrafts such as the B777 family, and to an extent, the A320 and B737 family as they received cockpit reworks. That said, decisions made on what plane gets reworked is not taken lightly, though the developers would want to hear the community’s opinion before making a decision.
One way to let the developers know what you’re looking for within the game is to vote in the Features category, and as you’ve mentioned, the C-17 is in need of a rework. Linked below is a thread in which you can vote, and hopefully, a reworked version can come sooner rather than later.