Zac Gallen, Sandy Alcantara, Jordan Montgomery, Jack Flaherty, Miles Mikolas, and Adam Wainwright
This one hurts even more than the outfield dynamic. If the Cardinals had never made that Marcell Ozuna trade, St. Louis likely has a top-of-the-rotation that includes the reigning NL Cy Young winner and the current front-runner for the award.
Jordan Montgomery goes from the club's "ace" to their number three starter. Jack Flaherty, Miles Mikolas, and Adam Wainwright could duke it out for roles in the back end of the rotation. I think there's a strong arguement for this being the best rotation in baseball.
Not only are the Cardinals no longer lacking an ace in this scenario, but the amount of quality starts and rotation depth they would have is incredible. We all know they need an ace badly, but the effect is not just in those important matchups where your ace is on the bump, but also the effect it has by "slotting" other arms lower in the rotation.
Something to note about most of this list - we aren't talking about guys who are currently on megadeals with other clubs. Gallen, Arozarena, Alcantara, and Garcia all make less than guys like Adam Wainwright, Steven Matz, Miles Mikolas, and Willson Contreas. The Cardinals could easily afford to have these guys on their roster.