If NHL GMs truly wanted their best chance to win they would get the best coach for the job.
This may come as a shock to you, but the person who gets hired to do a job isn't always the person best suited to do that job.
Pierre McGuire was hired to coach Hartford (despite having no head coaching experience at ANY level) and lasted all of six months, leading his squad to a 23-37-7 record (that's bad). He was fired and the captain of the team immediately started talking shit about him, saying it's the best thing that could have happened to the Whalers, and that the players on the squad had no respect for Pierre (that's really bad). He also had to forfeit some of his salary because he shared proprietary coaching evaluations with a different team (HOLY SHIT THAT IS BAD).
I guarantee that someone else could have done a better job. Somewhere on this planet, there was a person, male or female, who could have done better with Hartford than Pierre.
But going by your logic, that isn't true. Pierre was the most qualified person for the job because he was the person who got hired to do the job. Do you see the problem there? It's a tautology.