any team with success will get pressed against the salary cap and will have to move some good players out and replace them with cheaper options. so far the devils don't have that type of success, that all their players will have great arguments to ask for some extra cap percentage and players on elc are the last thing you move out. the devils won't have any urgency to move casey before the summer 27.
i wouldn't call casey untouchable like hischier, hughes, hughes and nemec. but considering he looks like a nhl player in the first year of his elc the devils would probably ask for more than most teams are willing to pay.
I'm not moving either but I'd be more willing to move Nemec than Casey. I don't think the talent gap between them is much at all. The trade value gap however is likely pretty large.