To me the top issues, in no particular order:
1. The inability to replace Dexter Fowler. The Cubs never had a good answer at CF or lead off hitter after he left. Neither the lineup nor the outfield ever really worked after he was gone.
2. The implosion of Addison Russell. An All-Star shortstop just evaporated.
3. Failure to develop any homegrown pitching for nearly a decade.
4. Repeatedly paying way too much in prospects and free agency money for short-term closer solutions.
5. Quintana not being the player he was with the Sox after being traded to the Cubs.
6. Stagnation and regression of young hitters after reaching the MLB.
7. General lack of urgency in the organization after 2018, coinciding with ownership shutting down any further big free agency signings. Inability or unwillingness to shake things up when things had obviously gotten stagnant in 2019-21.
I wouldn’t have the Darvish or Heyward signings in the top 7 of what went wrong. Heyward’s contract hasn’t been a constraint on spending for 2 or 3 seasons now, and he actually came reasonably close to giving his contract value from 17-20.