When you hear the question, "Should Team X add Erik Karlsson?" the answer is a reflexive "yes." You don't think about it. You just do it and watch the greatest defenseman of this generation play for Team X for the next nine years. In a vacuum, the question of who is untouchable in a deal for Karlsson is almost an invalid question.
It seems crazy to say the Dallas Stars should make anybody untouchable in a trade for Karlsson, but as of now, they are. Miro Heiskanen will likely not be moving anywhere. We don't live in a vacuum, or live in the NHL of the early aughts and before.