1. The team would like to add another big bat, but none of the FA a good match. Position of best fit would be 3B allowing Gyorko to assume an infield utility role where he always starts against LHP (be it at 1B or 2B) and fills in across the diamond when others are injured/rested or under perform.
2. SS depth is a worry and Carp's shoulder is a concern.
3. Greg Garcia doesn't fit the roster at all (bad defender at SS, can't platoon with Wong at 2B, bat not good enough for 3B).
4. The team with the player that is potentially available (Machado who would start at 3B and cover SS, allowing Gyorko to be the de facto cover in case Dejong regresses) wants young SP but we lack quality rotation depth (we have far better options at 6-8 than most teams but our back-end is a lot riskier than other contenders).
5. We could also use another good RP which BAL have in Brach (or O'Day if the Cards front office prefer him).
So why not take advantage of the slow FA market and sign Darvish?
Then trade Weaver, Garcia and a couple of others (Junior Fernanndez and another secondary piece, would be John Brebbia in my hypothetical as i think his value will never be higher) for Machado and Brach.
BAL know they're not getting 10+ years of control via two good SP for one year of Machado but here they get just about the top end of the type of player who would headline a deal and at the position they want. Losing Weaver hurts but i don't think Flaherty gets the deal done (plus i personally have them closer in value than the consensus).
In terms of 5 year WAR we likely lose the deal (a Machado extension or re-signing is a completely separate matter as he appears keen to reach FA and test the market), but due to where STL is on the win curve as well as being able to use the payroll flexibility we have to actually improve the projected strength of the rotation by signing Darvish we come out ahead.
Plus the depth that Gyorko gives us in this scenario helps to increase our floor and leaves the team less vulnerable to a drop-off via injury or poor performance in the infield (we are already well covered at catcher and in the outfield, plus have a number of fungible bullpen options that would begin the year at Memphis as well as Flaherty as our SP 6 and Reyes as an x-factor).
So if the talks with the Orioles previously broke down due to them insisting on receiving starting pitching when we wanted to move outfielder and if the market for Darvish is muted enough to make him a fair value proposition then the Cardinals can enact a one-two that would hypothetically put them above the Cubs. With Darvish off the market Chicago would lack the ability to make a meaningful counter-move as the drop-off to the next tier of SP is quite large.