Correct me if I'm wrong (I'm making a lot of estimations and generalizations) but it might be worth it to resign him. I think we now see the importance of the interior of the line. We addressed it on O-line and we have some potential on D-line.
At the moment, Melton is starting to get his legs back and Crawford is trying to figure out where he belongs. Despite the limited number of snaps, they seem to have been having success when Crawford and Melton are on the field at the same time (I assume this only happens in the Nickel). So, let's say we have 60 defensive snaps a game and 40 of them are in the Nickel. So they're on the field together for 40 snaps a game, plus they split the other 20 snaps in the base at the 3-tech (so they each get to rest 10 snaps a game). That means you have Melton and Crawford each playing 50 snaps a game (40 of them together) and then 20 snaps from whoever is at 1-tech.
If you look at it that way, it might be worth it to keep both of them. They have the potential to be one of the nastiest DT duos in the league. But we'll see later in the season. If Melton gets back to 100% and they're both on the field at the same time (and are both productive) then we'll have to consider keeping him next year.