You are wrong. Cassel isn't the problem in KC. He's not Brady, but few are. What he is is an above average starting QB fully capable of winning double digit games with a solidly built roster and competent coaches running a system he is reasonably proficient in.
Your problem is Pioli made a huge mistake in taking a shot on the golf pro being any more than a guy who got lucky to land a coordinators job on a team with a fairly loaded offense. His ego has exacerbated any shot Pioli believed he had at kick starting a rebuild in KC. Would have been better off in hindsight in hanging on to Herm as a placeholder until he could talk RAC and Charlie in off the street. Instead he's now on his third OC in as many seasons watching his hail mary loosly system related fall back HC candidate using up every inch of rope handed him to hang them both. Haley wasn't with them in Cleveland and he's one of two clowns (Tannenbaum being the other) that Bill had no interest in taking with him when he exited the swamp.
Pioli got tired of riding Bill's coat tails and wanted to prove he could replicate that level on success as a stand alone GM. KC is his Cleveland...and Haley's. Chief's fans don't have a clue what the problem really is... I guess it's easier for them at this juncture to just point fingers at the QB none of the pundits could stand to admit is at least the best franchise prospect to ever come out of the USC program...largely because he never played in it and developed an entitled mentality.
It's only going to get worse in KC because Pioli doesn't have a HC on his staff who can cover for some poor personnel decisions (like the punk WR) and some really bad luck (like losing Moeaki who is as talented as he is injury prone and Eric Berry). Chief's players don't have a bonefide coaching genius to look to for leadership in adversity, they have the annoying golf pro...).