I think that's an oversimplification. It's not only about today's #3 vs #8, it's about the future. Yes, #3 is signed for three more years, but he's also much older than #8 and #8 has presumably not reached his potential yet. There's also salary and salary cap implications to consider. #8's value is increased due to his age and his next contract earnings being far lower than #3.
For me, there are merits to protecting both guys, and there are merits to exposing both guys. At this point in the season, it's a toss up, but we're not quite half way though the season. Stanley may not play another game this year and be demoted to the Moose, or he may play and play terribly, lowering his internal value, while DeMelo could return to form and become the solid #3 we hoped for when we re-signed him. There are many variables to consider, it's not just simply #3 vs #8.