You keep Biagini in AAA because there's a pretty good chance you'll need a starter during the season (if not in April), and the other depth options have barely pitched above A+. Having him stretched out and working on his repertoire as a starter seems a hell of a lot smarter than restricting to a 7th inning role where he'll throw an inning at a time, and then having to stretch him out slowly after that. If he was out of options, then that's a different story, but he's not. We also saw how his performance suffered last season when he kept changing roles. He couldn't get outs as a reliever after his starting stint.
The Jays still have time to sign someone to pitch out of the pen (Oh!). They also have a s*** ton of options who might be able to win spots out of spring training. We are not looking to replace Osuna, we are looking to replace a rule 5 pick with one good year out of the pen two years ago. Wouldn't shock me at all if some of the internal options or minor league signings can come in and take his spot in the pen.
If AAA had more starting options then I would agree to just keep him in long relief and stretch him out when needed. But depth is a huge issue for this team, and even though I agree the only chance this team has to contend is if 4 of the 5 starters stay healthy, that doesn't mean you short change the depth.