1) Even if you are going with the argument that he was "no worse" than Holden or Girardi, the veteran is always going to win out in that situation for damn near EVERY coach. For a kid to win that battle, he has to play markedly better than his competition.
2) He is NOT good at outlet passes to start the rush. He IS good at passes to obtain possession out of the scrum, but anything longer than 15' is not his thing. (At least not yet.)
3) He IS worse 1-on-1 than Girardi and Holden. He gets flat-out beaten more than anyone who's suited up for the NHL squad in the last two years. He did show some improvement during his longer run last year, but has since regressed, while Girardi has improved.
4) Also, when Girardi and Holden do get beat, they usually manage to at least hamper the other player. Dylan is phenomenal at laying guys out, but when he misses, he misses completely way too often, and lands on his ass/knees way too often while doing it. (Though to be fair, Holden when playing his off side does get himself out of position far too often, which while not technically "getting beat", does lead to open players.)
5) Barring a string of 2-3 injuries on D and no trades (which we all think will happen), he isn't going to get into enough games to remain an RFA, so odds are he'll be gone at year's end any way you slice it.
If he wasn't a fighter and/or former 10th OA pick, no one would be as fussed as they are about a player like this. I really do hope he is included in a trade, because I hate losing assets for nothing, but as Leslie said, this is hardly mismanagement.