The most important thing is that it's really, REALLY stupid to care about the bullpen on a losing team. Most relievers are great one year or for a couple of years, then they suck for a few years, then maybe they get it back, or not.
Now that doesn't mean you don't continually draft and develop pitchers so that you have a wealth of young pitchers to restock your pen with. And I think Rizzo is showing, both through who he targets in trades and who he drafts, that he's committed to having a ton of young pitchers around to choose from.
But the value of Matt Capps is completely zilch to our team. He's doing well this year, but our team sucks. Even if our team is better next year, he could easily suck (just like he did LAST year).
So you never keep relievers on a crappy team, instead of trading them for prospects, because you hope that they will still be good when your team improves. The odds are not with that bet.