It's very true it's case specific. I used to have conversations about this in general terms with an acquaintance but I presented here moreso as it pertains to the Cubs since he was kind of drilling into the Cubs pitching staff.
But yeah, when we'd discuss this, I would say a good bullpen is worth a good (not great) # 3 starter. He said a #4. But this was before the playoffs last year. There's a lot to be learned from last years World Series including where this topic is concerned. The Royals had one of the best bullpen tandems in the history of the game. It held up its end in game 7 but, in the end a stud #1 (eg B'garner) proved to be more valuable. But a #1 who pitches up to it, not like what happened with Kershaw. And maybe a second #1 (as your #2) like Greinke. But beyond that, it's hard to put a higher value on something more than that KC bullpen. There wasn't one pitcher in the Royals starting rotation that was more valuable than the Royals bullpen, except maybe Ventura. And that's a big maybe.
Few teams have pitching like what you see in the PO. That's typically why teams go far in the playoffs. So, can you take this exercise of comparing the Royals bullpen to different aces, can you then put it to scale when evaluating the non playoff teams. In other words, if a team has a "good" bullpen and "good" 2s, 3s, and 4s, does the ranking end up being similar? It might be skewed since KCs bullpen was such an outlier.
I think this is an interesting discussion. I feel if someone has a solid handle on this, it could provide insight into where the game will go in the future. You sometimes hear about, down the road, teams going to shifts (like hockey) where over a game, one pitcher gets one time through the batting order. Perhaps it will. Or perhaps it will only on teams that lack a true ace. But to get a sense of it, starts with properly valuing the relative importance of each.
