I doubt either are off the Leafs at the beginning of next season.
Management said they'll make it hard for rookies to break onto the squad which is fine. I'm fine with it if neither are on the big team this year, so long as they don't deserve to be on the team.
But Babcock has 100% stated that if a player is good enough to steal a good spot on the team. They will be on the team. And honestly I have a very difficult time seeing Nylander not being better than most of our forwards not named Kadri/JVR/Healthy Lupul this year. If he's better than everyone else he won't get sent down. Babcock said this.
And no comparable caliber of prospect (Especially a forward) as Marner. Has ever been outside the NHL for more than 1 season. Unless stuck in contract in Russia or Europe.
I'm all for patience. But Leaf fans are going overboard with it. If they had their way neither would play in the NHL until they are 23. Even Dylan Larkin has a real chance of making Detroit and he's 19 and not as good as Nylander (Although very close).