duffy and ventura hit walls at like 80-90 pitches. they did all last year, and that isn't going to magically change just because shields is no longer around and you want them to pitch longer all of a sudden.
i don't think guthrie and vargas have endurance problems, hitters just figure them out after a while.
whatever the issue, volkie (maybe, dunno anything about him) is the only one that should be routinely expected/hoped to go deep in a game. get the others out while the getting is good.