The thing with Casas is, he strikes me as the kind of guy who is going to bet on himself, in otherwords it might be really hard to keep him away from going to free agency.
Obviously we want to see the Sox resign more of their homegrown talent, and they really need to do that. But even when you operate under that mentality you're still going to lose some guys to free agency, you can't resign everyone. Casas would likely sign a deal buying out free agent years under the perceived value that he thinks he can grow into over the next 6 years, he'd easily be worth it if he is that player, but it also means more risk up front for the Red Sox.
This is pure conjecture, it will be interesting to see if he's extended at some point.