No, the opposite is exactly true. In the 70s, people follow their team through the newspapers. So, translation is fine back then because they READ about what the coach says; they don't HEAR it.
Nowadays, the situation is different. Modern technology allows people to follow their team, not only via newspapers, but also on TV and the Internet. These two mediums means that people now have the ability to not just READ but HEAR what the coach says, and they can also hear it in real-time. When you want to HEAR someone say something, then translation sucks. You want to hear the original language, listen to the tone of voice, whether he is happy or frustrated or sad and so on.
Back in the 70s, the coach only needs to speak to the players. Nowadays, he does that AND does public speaking. Since the majority of Canadiens fans are francophone, it's a personable touch to have the coach speak the same language so that the francophone fans know that the team represents them (which it does because it plays for MONTREAL and since it is the only team in Quebec, it also plays for Quebec). Duh!
I am an anglophone and I am fed up with anglophones (especially Canadian) constantly bringing up the language issue. It is a common practice in European soccer leagues to have the coach speak the local language so why not here in Montreal and Quebec despite the fact that French is one of the two official languages of Canada.