There's also the fact that Galchenyuk was doing what he was doing in what appears to be a pretty toxic locker-room environment in Montreal, where you had GMs and coaches constantly undermining their own players.
Imagine if Bill Peters was to come out during a summer golf event and say something like "well, it's pretty obvious that Sam Bennett can't hack it in the NHL as a centre," or if Treliving last week had responded to the leadership question by saying "yeah, we definitely don't have the best leaders, and that's been holding us back." That's essentially the stuff that Montreal has been synonymous with over the last few years.
Domi still has a ton to prove in this league after what happened last season. And I don't think he's going to a place where it will necessarily be easier for him to prove himself (see Drouin, too). Galchenyuk has a 30-goal season in his back pocket, and I think he definitely is going to a place where he'll be more comfortable and more able to focus on his play.