The fact is that you have to assess his bullpen WITH that caveat, because it exists as a real fact. Bullpens are the last fronteir for most GM's, it's really difficult to build one that's consistent from year to year because a handful of bad games can inflate a reliever's numbers for entire seasons.
That's why the question you ask is a lot harder than you're pretending it is. Because all bullpens are inconsistent to a greater or lesser degree, and even when you get the chemistry right one year there's no guarantee that exactly the same mix of guys will get the job done next season.
Realistically all you can do is go for the best talent you can without giving up major assets for relief pitchers (which is stupid) and that's what Theo did, and then when it doesn't work, ask what happened and whether anything could have been done to prevent it.
What happened was that 3 different relievers we had no reason not to depend on going into the season completely regressed and became worthless. That is *not* something Theo could have planned for, and having the depth to absorb 3 tank jobs like that isn't something any bullpen's likely to have. You might have gotten some mileage out of replacing one of them with a quality setup man, but playing catch up with 2 very good teams and already 5-6 games back, Theo made the judgment call that it wasn't going to be enough and the team didn't spend the assets it would need to try to force itself into contention, waiting instead for the blank slate that is the offseason.
THAT is the decision you refuse to understand -- because you have no stomach for a bad season. THAT is what you want to blame Theo for. For making the decision to save his ammunition. The rest is just you trying to dignify your position by blaming him for the lesser things that led up to his decision.
Combine the issue with the Pen with the unlikely injuries to Pedroia and Ellsbury which probably will not repeat next year, automatically making both our pitching and our offense better next year, and the mess at catcher, and I think it was the right call. The fact that we lost Youkilis almost immediately after he made the decision not to trade for Downs kinda ratifies it. It just wasn't going to work out this year, end of thought.