Exactly, because if he has a high trade value with other teams, that means he's likely doing well for us, and if he's doing well for us then why trade him? Even if we have 6 amazing starters so that we can trade Lackey, a team would never give us the best package for Lackey, so once again it wouldn't make sense to move him.
The only way it makes sense to trade Lackey for prospects is if you have an opposing GM that's completely retarded, which I find increasingly rare as the years go on.