You could make a pretty good case that Chapman could realistically provide 3-4 wins from the pen. Running him out 110 innings instead of 70 while greatly reducing his effectiveness and the average leverage of his innings is probably not worth it.
Who cares what his FIP projection is? And even if you did, I'm pretty sure that Steamer isn't considering that he's a knuckleballer when projecting decline. Basically, projections are worthless for Dickey.