All good questions, but I will counter with this. When one is fatigued, he doesn't think or execute as crisply. No one will gripe more about a lack of fundamentals than me, especially as it regards tackling. It is difficult to quantify the degree to which exhaustion affects the level of play but it unquestionably does. Just watch the difference between a gassed fighter vs his early rounds. It would be nice to have that kind of defense that plays with resolute effort keeping an impotent offense in the game until the bitter end. I don't pretend to know how much of this is mental toughness, conditioning or the degree of coaching and reps these guys are getting on fundamentals.
Just one example though -- BJ Foster. Someone wroteup how he was an exciting assassin to watch his freshman season, and has regressed ever since. Well, yeah, he was aggressive and a great athlete. But he never tackled correctly. He was always hunting the big hit with his shoulder bump or missile launch. To__ Orlan_o did him no favors. Mr. Just Put Your Foot in the Ground and Swarm to The BAll. OK. Great, yes, lots of bodies towards the ball. See ball, or what you think is the ball (misdirection/fake handoff anyone?) and everyone swarms to it and increases the odds of getting the guy down. But no fundamentals. Is BJ too stupid or stubborn to learn? Has anyone ever really made a solid effort to emphasize proper technique? Are his bad habits just too ingrained? Wish I knew the answer.
I think it's far too early to call for either Sark or PK being changed out. I do have some nitpicks and concerns. Keep in mind though, that when there are a lot of things to fix, you can only fix so much week-to-week.