Free agency isn't a guarantee to get what you want when everyone other team on baseball (except a few ) are also actively looking at those same relievers. It's also even more likely you end up with a multi-year deal with a volatile asset dragging on your payroll, or just plain hurt and contributing nothing... a true sunk cost.
I've been looking at all your replies and I understand the premise of what youre trying to say, but there are several assumptions about this team's ability to just pluck talent out of thin air and develop it, which makes it (in your view) inefficient use of prospect capital to acquire relievers with control. You'd rather the Jays gamble on the teams ability to attract and sign top FA relievers when available, or develop more internally.
Problem is... the Jays havent been able to develop any relievers consistently on a year to year basis in the past...decade until maybe the last 2 seasons with Fluharty being the only true member of the current pitching roster fully developed internally. Yariel was already a pro Cuban pitcher so I don't really count him. Little was a successful reclamation project, so thats good too so I think its trending in the right direction, but they still aren't a pitching factory yet. Fisher is a good sign but his development was like 95% in the Dodgers system.
So, today, I submit it makes more sense for the Jays in their current situation to not purely look at rentals and still grab a great looking controllable reliever when they can, even if the value comparison in a vacuum tilts to the other team.
If they become a reliable franchise capable of churning out home-grown relievers on a year-to-year basis, rental additions make far more sense