Fair enough but then it's pointless to hire him.
You don't hire a coach to be the assistant to a guy you have on a short leash thinking he'll take over if the first guy fails. It's just a terrible situation for people to be in, so there's no reason to think that's what Bergevin is thinking (If this rumour is even true).
So then it comes to thinking Boucher can help as an assistant. Either it's yes, the team does better and Therrien's job is safe, or it's no in which case you probably fire the whole staff, or do nothing because somehow that's an option in Bergevin's mind. If it's the latter then Bergevin probably gets canned, and the whole coaching staff so if you want Boucher as HC, this move probably wouldn't help.
Best case is Sly gets promoted, Boucher takes over the farm team, and after the firing Boucher gets promoted.
But like you I don't think they want a guy who thinks outside the box like Boucher, they seem to prefer yes men.