I don't think Cup credibility is of any consequence when it comes to coaching. Just because Roy won a Cup 30 years ago is of little consequence to players, many of whom were not even born when he did.
What matters is coaching ability, ability to put players in positions to succeed, establishing a system, etc., etc., etc.
Players are caught up in there here and now: in their own performance, where they are in their careers, how the team is doing, etc.
I love Messier, but do you think he would be a successful coach? Yeah, everyone knows his record and persona....but would it matter in game 35 of 82? Or when a team is down two goals going into the third period?
The only questions with Roy should be: can he coach and bring out the best in players, is he open to new ways of looking at the game, is he open to the new structure of management where GMs have more say in line-ups, does he have the personality to deal with 23 players of differing temperaments, different motivation points, etc.?
He might be able to "overpower" impressionable 16-18 year olds in the Q., but that is not going to fly when it comes to dealing with men.