First, nobody said they weren't a liability. But the offense scored 13 points against Baltimore vs. a secondary missing 4 of its top 5 players, 14 in the "I own you game". They only scored 6 in the 1st half vs. Pittsburgh and a special teams TD was the only reason they had a chance in that game. If that's the standard for "defense blew the game" then your standards should be higher for the offense.
Secondly, how many more years do you have to see the Chiefs, Packers, Chargers, Bucs, etc. score 27+ per game to realize 18 ppg isn't good enough? The secondary has such a small margin for error because the offense can't score. You don't think the Chiefs blew coverages among their 253 passing yards allowed per game? Hell, Buffalo almost beat them in the playoffs on a blown coverage, can't get any more costly than that (unless you're Chris Conte). Shit happens. Bad coverage makes a much bigger difference when you can't win if the other team scores 3 times.