Take away Bieber and you only have 5 "MLB caliber" pitchers (including Allen), 2 of which have bounced around the IL including one who has been pretty terrible so far. So yeah then you're looking at guys like Battenfield and Curry as you mentioned who are a terrible downgrade over Bieber. This rotation is stretching it thin WITH Bieber, let alone without him.
We can think of how much we want the 3 kids to be pitching but if you're already waving the white flag in the playoffs, then why would you even want to "ride with the rookies"? and get them to those IP counts, particularly someone like Williams? And Williams threw 5.1 IP/start in AAA with a BB/9 of 4.1 so expecting him to provide the volume going forward seems extremely unrealistic.
Without Bieber, there is no room for basically anything wrong to happen, teetering on a tightrope. With Bieber we can still end up with Battenfield, et al. If you are trading someone like Bieber and not adding MLB pitching at the same time you're better off just getting prospects because you're already waving the white flag.
You are missing a key point that I made ...
"Without McK, this team isn't set up for long playoff run whether we have Bieber or not. Trading Bieber has nothing to do with innings for the other 4 starters."
Trading Bieber for me has nothing to do with "playoffs" as to make playoffs
1) As you said, everything has to go right -- healthy Bieber and Civale
2) We need to ride our rookies as you don't want a serious downgrade with Battenfield, Curry or others like Gaddis -- even as a piggyback - as it is planned vs just pulling guy early to save innings when we are down
3) So for me, a playoff, is we "ride" with our best rookies Bibee, Williams and Allen as our 6th for those what ifs.... then my point, at playoffs they are at 150-160+ (with Williams high of 110 and Bibee and Allen at 132 high) ... so they can go 1 round in playoffs but not this "We have a chance for WS" as no one is left for innings.
4) Having McK back in September is the only way to make playoffs and go past round 1 and reports of having him back are a hope and prayer at best