I get why people want to rag on the bullpen but honestly still think the issue is the fact that the cubs are often terrible at hitting with RISP. They aren't capitalizing on opportunities throughout the game which keeps games tighter than it needs to be. For example, last night they were 0-7 with RISP and they left 11 guys on base. If you contrast that with when they score big like say the 9-2 game in cincy, they were 3-9 with RISP and they left 8 on. The 7-2 game vs cincy they were 3-11 with RISP and left 9 guys on.
NL "average" batting average is .259 this year. Only Bryant, Szczur, Ross, Contreras, Almora and Rizzo are above that with RISP. You also have a few guys like Russell, Fowler and Zobrist who have really high OBP with RISP but they often have trouble getting that one hit needed to break things open. Overall as a team they aren't really "bad" at it as they hit .266/.381/.461 which if you go by batting average is 14th best in the majors or if you go by OBP they are 3rd. Also, Baez can't hit RHP right now though he's been quite good vs LHP and mean while Russell is the opposite. Montero can't hit anything and Heyward hasn't hit for average this year. That makes managing around jams a lot easier for an opposing manager.
Ultimately, bullpens blow leads. That's just the nature of things. In my opinion, you don't fix their issues in close games by addressing the bullpen because no bullpen is ever fail proof. You fix it by figuring out how to score more runs. I've given this a fair bit of thought and I don't exactly have the most solid answer on how to do that. It's tricky because you want guys like Russell and Baez to play in order to grow as players. So, sometimes you just have to live with their struggles in the hope that down the line it pays off. Plus both Baez and Russell provide a lot on defense and the same goes for Heyward. That said, playing high average types like Almora and Contreras should help alleviate some of those issues.
And don't get me wrong, I'm not saying they shouldn't add to their bullpen at all. As I've said for awhile, I think they need a lefty specialist like Abad. All I'm saying is people weren't noticing issues when they were averaging 6.18 runs/g on offense in April. When they've dropped to around 5 a game in May/June it's become an issue. The cubs lead the majors in OBP at .360. They are 15th in batting average. If they find ways to get higher average guys in the line up be it platoons, trades.... whatever, they're gonna score more runs and close games are less of an issue.