The Osuna situation is such a mess. Even if he successfully transitions to starting what happens if the Jays make the playoffs? He'll surely be approaching whatever limit they impose on him. Put him in the pen?
If they put him in the pen this year I think that signals that they're married to it. Then you're basically hoping you can get 6 years of above average BP production out of him before he becomes a FA. If you don't, it's a waste of a a good SP prospect.
I know we say this all the time- but here is a guy who's value may never be higher. If they don't think he can start maybe he's best used in a trade.