The problem is that a 34 year old goalie with year-to-year consistency issues and two years of term left is an enormous risk, and nobody is going to trade for him unless that risk is minimized in some way. I get that he's having a great year right now, but buying high on a goalie on the wrong side of 30 could have devastating implications for a cap team trying to compete.
Getting a team to absorb Markstrom's full contract with no conditions would mean a huge concession by the purchasing team, which will greatly impact the return he gets. Calgary won't get a premium return unless they retain something, and if they aren't retaining then the return is significantly diminished. Teams will look elsewhere. He's old, expensive, and not a long-term answer - there are goalies available with far less baggage.
NJ could accommodate Markstrom's full cap hit this year because Dougie Hamilton is on LTIR and they have an extra 9M cushion to play around with. But they would need to move out significant salary in order to be cap compliant next year, which is a huge issue considering how many young RFAs that need to be re-signed in the short term (Mercer this year, Luke Hughes and Holtz next year, Nemec the year after). They also need to bring in a replacement for McLeod.
I suspect the Devils won't be very active at the deadline - maybe a minor move here and there, but they'll probably just stand pat and let the chips fall where they may. Perhaps they go on a late run and make the playoffs, perhaps they don't. They'll fire Ruff this April and likely make a goalie move at the draft or over the summer. The skater core has proven it's very good and can win a lot of games, but between injuries, coaching, and goaltending this just isn't their year. The team is still young and in it's prime, run it back again.
But the Devils just gave Fitzgerald a huge vote of confidence in naming him President, they are all-in on his long term plan and will not alter that for a panic move one year into their window. If there is a good hockey deal to be made then he'll make it, but he's not going to get squeezed - if he was, he would have done something by now.