It's all about VORP. Brady, Gronk & Revis have ridiculous value over whoever else would be in their place. Arrington's value over Ryan or Dennard in the slot is substantial, but it's a fraction of what Gronk over Hooman is or Brady over Garap or Revis over any other CB.
You can withstand a few injuries where you lose some value (Mayo, for instance) - or maybe they pile up to a breaking point (2013). But as long as the latter doesn't happen, but as we saw in the championship years, we were able to weather losing some very good players as long as enough of our key guys were healthy.