Your larger point has merit but if we do sign up Rockliff or Motlop (or some other free agent) we wouldn't receive any compensation for the loss of Pittard/Trengove. Clubs who go one-in one-out in free agency don't get a compensation pick.
Realistically what would we get for either player? At best a late 2nd rounder and this draft isnt as deep as last years thus why we traded to get a few extra 2nd rounders.
Ideally we make a play for Rockliff as its a free hit and we supposedly have the cap space. Apart from that there is no one else worth getting unless we go for motlop. Casboult would suit our structures but may not be worth the money to get him, plus by next year dougle and frampton should be ready.
We can afford to lose trengrove if he decides to leave. Would prefer Pittard to stay as the compensation wont be worth it, whatever we get.
We can lose guys like toumpas, lobbe, gus and white which should free up more cap space. Any other players like snelling, krak, gray, young, achee can stay for depth unless we need the spots.
Eddy and lienart can be moved on so we can look at some rookies, project players or mature agers.
Apart from that we wont get much for players we dont want like hammer, young etc so its worth just keeping senior players and looking at the FA market. If we have the cap space id like to get rockliff and hope to have another good look next year on some FA's.