Yeah the Leafs are obviously in no rush to move along the prospects. The Leafs have 24 players on their main roster now. 1 of them will go down to the minors (probably Casey Bailey since he's waiver exempt and lacks pro experience. The other 23 guys I think are all multi-year pros with more NHL games under their belt). They stocked the cupboard with hungry vets on 1-2 year contracts who are seeking redemption and/or have something to prove. And this is by intentional design. They want to make it difficult for the prospects to make it onto the Leafs' roster. And they want to make it difficult even for the vets to get ice time in order to force them to work hard and compete. Babcock and management don't want Leafs' players to be friends of complacency.
If Mitch Marner and William Nylander are not good enough to make the Leafs' Top 6, they shouldn't be playing in the NHL. Marner projects as a future first-liner and Nylander a future Top 6 forward at least. If you play on the third-line, your offensive production doesn't have to be as good. But you have to be a decent forechecker/backchecker, etc. at least. If you're on the fourth-line, you need to be good at defense. Basically the less value you provide offensively, the more value you have to provide defensively. It's important for hockey players to play to their strengths. It's more productive for Top 6 projected guys like Marner and Nylander to hone their offensive skills in the OHL and AHL respectively than to play a game they are less suited for in the NHL (bottom 6 hockey).