In a nutshell, that it might be better for him to go back to Junior and you think he's probably not ready for the NHL (mostly citing the WJC as a demonstration of that). That's about it right?
While I think that Marner IS ready for the NHL and could definitely put up 40 points this season (which is great for a rookie) even if he has a slow start. And I will concede there is no way to know for sure if he's ready or not without seeing him actually play in the NHL, but even if he has an average showing at camp, he should still make the team.
1) Training Camp shouldn't completely overshadow all of his other achievements; even the oft-cited WJC tourney where Marner wasn't his best self, he was one of the best players on his team and had an absolutely dominant stretch when the entire tournament was on the line for Canada.
2) He is an elite asset that has a very high chance at being an extremely dynamic and productive NHL-er and therefore his development is a bigger priority than what he deserves based on camp. I also think it's ridiculous that people have such a black/white way of looking at that. 'He has to dominate at camp to make the team, he has to earn it' is a very narrow way of looking at the situation. Mitch is a good kid and a hardworking player with an incredible set of raw skills who achieved more than most players his age get to. He is not like Brown, or Sosh, or Hyman, or any of those guys. If he had attitude problems and was prone to being lazy, then I'd be wary. He isn't that guy though.
3) is the NHL the ideal situation for him? I don't think so, I think it would have been perfect if he had the opportunity to play in the AHL like Willy and get called up sporadically. Unfortunately that's not an option, so the NHL is the next best league for his development.
I don't really see how you can see it any other way to be honest, when you watch him play in the OHL it's clear that he's on a completely different level than 95% of the
players there, if not more. And we need him to constantly be learning.