You actually sell LC a bit short. He also left 2 very good OL behind, Schilling and Boren and a couple of decent (when not injured) RBs. Let's assume Mallett and Arrington would stay (Although things written on Mgoblog about RM's relationship with the coaches and his teammates make that questionable and I think AA was going to leave since his stock was unlikely to go higher than it was after his incredible bowl performance).
So, whoever coached in 2008 was going to have an offense with two linemen, a good but immobile QB and three possession WRs. Manningham was gone in any case and Carr gets the blame for not replacing him. That offense was going to be terrible regardless whether LC stayed or Les Miles (our savior!) was the coach.
You're absolutely right that the starting DL and DBs were good (and Brandon Graham is one of the all time greats), but there was little depth and the LBs were awful.
To sum up: an awful offense combined with a defense that is at best "decent" leads to a losing season - and that assumes that Mallett stays.
No coach could have succeeded with that team.
Side note: it's amusing to see Stevie Brown listed as an asset left behind by LC. He was considered a joke on this board before RR's staff found him a position and coached him up.