Even if Marner comes in and scores a hat trick in every single preseason game, it's still the right move at this point for them to send him back to junior in my opinion. The risks just outweigh the possible benefits given his size/age and the point they're at in the rebuild. Marner may have a high compete level like Landeskog, but Landeskog was also the most nhl ready prospect according to most scouts because of his size.
Based on everything management has said about developing prospects the right way, I can't see any way Marner even gets a single regular season game this year. Let's face it, this year is going to be another gong-show full of losses. No matter how young he is or how bad the team is expected to be, the TO media/fans would still expect huge things out of him and would start calling him a bust 20 games into the season if he doesn't pile up points. It's not fair, it's just the way it is here. Let him continue to dominate the OHL and play in a winning environment for now. He may not have anything else to prove there, but it shouldn't hurt his development. Having him struggle at the NHL level and have the fans turn on him or getting injured could hurt his development. We can still ruin him in two years from now.....there's no rush