They didn't. You are misremembering something. They never allowed that many yards again. The defense markedly improved the year after Spags left, then dropped off the next 2 years. The thing that you may be thinking of is in Rob Ryan's 3rd season, he gave up more points per game than Spags' season.
I'll have to dig around but I feel like '14,'15 or '16 were all really close to '12 if one didn't surpass 2012. It all blurred together as a run of all time futility.
Point to me was, we kept being bad. Spags moved on and returned to being pretty good, good enough to win Super Bowls.