Nylander is a whole three years younger compared to Miller. That isn't enough to make a difference, when we are already considering trading players younger in age in Boeser, Garland (10 days younger) and/or Hoglander. One extra year of control is, according to Seravelli anyway, not worth much, if teams are expected to offer similar packages to what was offered already with one year remaining on Miller's contract. One year extra isn't something we'd be worried about, at all. Nylander is an inferior player in nearly every aspect of the game, and an extra year of control and being three years younger is not enough to overcome this.
What a team does with Miller after trading for him is none of our business. If Miller doesn't like the new destination, or team composition, or management, and chooses not to sign, that doesn't affect us in the slightest or what we expect as a return.