Of course we make mistakes like every other team. We've made some big ones IMHO, some of which you've cited - the failure to address the LBs in 2005-2007 or the secondary last year, for example. Some trades and draft picks have been horrible in retrospect, such as Chad Jackson and Starks.
Could this trade be a disaster? Absolutely. I personally don't think so, for a couple of reasons, but I could easily be wrong. My reasons:
1. BB was not making this trade out of duress (such as the Deion Branch trade), and he would not gut the DL of a SB contender unless under severe pressure, so I think he has confidence in the other guys on the DL, and he thinks that with the schemes he will be running Seymour's contributions may not be missed as much. BB is fallible and could be wrong about this, but I don't see him making this kind of move if he felt the DL would be severely affected. I think he has a lot more confidence in this defense than most people on this board.
2. Seymour was almost certainly gone after 2009 anyway. While MgT and others can rationalize that we didn't get much more than we would have if he had become a FA (e.g., MgT's argument that we are getting a 2011 1st rather than a 2010 3rd comp which = a 2011 2nd, or that the 2011 1st is only the equivalent of a 2009 3rd) I think that is bogus. There's at least a reasonable chance the 2011 Raider's pick will be top 10 - no guarantee, but given that they've picked 7-4-1-7-7-2 in the last 6 drafts it's not a bad bet - vs. a 2010 comp which would be around pick #97-100. No comparison.
I don't think I'm "chugging koolaid" about this move, but I'm ok with it. It's a calculated gamble, and one which could backfire in the short term, but I suspect we'll be ok.