No, as much as I'm skeptical about what I've seen from him this season, McNabb was objectively VERY good last season, by every metric and even by the eye test, ESPECIALLY considering he was tossed right into the fire of a craptacular season. He SHOULD be a good 2nd pairing dman, not just serviceable; the org has great faith in him as they did in Muzzin. I'm worried about him taking a step back, sure, but let's not minimize what he did as if he was one of last year's problems. He was one of the bright spots.
Edit: last year he was second only to Muzzin in CF%, 4th on the entire team in GF%, the best GA/60 of all regular defensemen on the team, all while in and out of the lineup, in a new system, with a partner-go-round. And that's just a cursory glance. All of this last year is why I've expected more this year. And as far as why 'that explains no playoffs,' his metrics were good enough to be a #2 defenseman on just about every other team in the league.