One thing we shouldn't forget about Primeau is Charlie Lindgren. For years, he was an AHL goalie who would get cups of coffee in the NHL. It's been two and a half years and he's now a full time NHL goalie.
I'm not sure why you think we need to remember this...
Are you suggesting Primeau should be retained until he's 30 because he might become a decant NHL goaltender at age 30?
Lindgren took a step in his late 20's, after he he had left the Canadiens organization to sign in St. Louis. The Washington pro scouting team definitely saw potential in him to be NHL ready and they made a shrewd move and lock in a solid net minder of a three-year deal.
But I don't see the cautionary tale here, and I don't think the Canadiens letting him leave the organization was the wrong move at the time. I was largely indifferent, if he had been retained, I wouldn't have objected but I also wasn't concerned when he left.
Primeau clearly outplayed Lindgren in Laval, and McNiven was better than him before that. He never was able to take over the Laval net and run with it because other guys were better.