This is basically what we'll be doing next year, the tweet is kind of misleading because the guy misinterpreted what the Eagles were doing with their personnel by counting DeJean as a pure CB, as opposed to the S/LB hybrid he actually is, which is exactly what the "big nickel" thing is all about.
So the Eagles would've been certainly top half of the league, if not top 5-10.
But the way the Seahawks used Emmanwori, the way the Texans used Pitre, and the way the Eagles used DeJean is almost exactly how we'll use Downs this year. I think Downs will play a bit more traditional, 2 high-type safety in base packages than Emmanwori or DeJean played last year (although DeJean is moving full-time to "safety" this year), but for the most part we're going to be in nickel with Downs functioning as a S/LB hybrid in the slot with Thompson and either Hooker or Locke playing more traditional safety roles.
The beauty of it is you can be in nickel with added coverage ability due to having the S/LB hybrid on the field, as opposed to a pure 3rd LB in base, but without sacrificing too much defending the run if the S/LB hybrid is effective supporting the run, which guys like Emmanwori, DeJean, Pitre and Downs are.