From your link, Joe, KC is 11th in the league in offensive Time Of Possession per drive - so their offense is doing a top-third job at giving their defense time to rest. They are also 4th in turnovers per drive, which also assists their defense, obviously. Of course the defense is carrying the offense - the defense is the better unit. But the style of the offense contributes to the defense's success. I think that an offense like, say, Denver's (3rd in yards but 23rd in time of possession, each per drive) would absolutely have a negative impact on the defense. There is a balance in effect.
And as others have stated, the biggest issue(s) with the offense are not at QB - it's the constant penalties and drops that have really set them back.