I don’t think that this would be an absolutely terrible trade for either side, especially since Dallas has fairly weak offensive depth outside of their top-line. However, I can see why Stars fans are adamant about keeping Heiskanen, and I think that there is much more than a passing chance that this “magic bean” will excel in the NHL. Heiskanen is the second-best defensive prospect in the game after Dahlin, their age and respective resumes showing that they’re not wildly far off from one-another, and Heiskanen has absolutely nothing left to prove outside of the NHL. Conner had a great season for the Jets, but the fact does remain that he played primarily with two of the best forwards in the game. I don’t really buy into the whole narrative of Conner making his line mates exceptionally better, since the team was excellent from the top-down, and Scheifele/Wheeler maintained their production in the playoffs while Conner did not stand-out offensively. Namestnikov also made Kucherov and Stamkos look good on their line, but it was entirely circumstantial (not saying Vlad and Kyle are comparable players, but elite talent will elevate other players, not necessarily the other way-around).
The question for Dallas ultimately boils down to whether they would want a forward group of Seguin, Benn, Radulov, and Conner; or rather a Defensive core of Klingberg, Heiskanen, and Lindell. The former would be helpful to the goal of patching larger holes on their offense, but those issues run much deeper than a single player could fix, and if Conner isn’t able to carry an offensive line (which seems to be as much of a question mark as whether or not Heiskanen can carry an NHL defensive pairing), then the trade holds significantly less value for Dallas. This error would then compound ten-fold if Seguin does end up leaving in FA. It would seem wiser for Dallas to hold out hope for Dellandrea to develop well, and for the probability of Seguin re-signing and helping to usher in a new crop of home-grown talent ahead of an elte defensive core.
At the end of the day, acquiring Conner isn’t going to be a magical fix for Dallas’ depth-woes on offense, but Heiskanen growing into a bigger role on the back-end alongside Klingberg and Lindell could certainly provide the ancillary benefit of boosting the offense ahead of them by having at least one of the three on the ice for the vast majority of any given game. Conner is a great asset in his own right, but the stakes for him not producing like Heiskanen might in a different role (and vice-versa) could very-well spell disaster for Dallas, so I see them standing-pat as the best option here.