There's a difference between dropoff and impact though. If you put a cornerback who allowed a 55% completion rate in Darrelle Revis place the difference would be about a completion a game. It's hard to see how that would be make or break for the Jet's defense over the course of a season.
If he has a monster year and tests Free Agency, there's a chance we may not be able to afford him next season. I'd love to see him on this team for a long time though.
My main issue with Revis is that I think he's too good at a position that is of fairly low consequence to be affordable by a good team over the long run. I'd love to have him at CB if he wasn't going to cost us other players at positions of higher impact down the road. In an uncapped scenario I'd love to keep him forever, but I don't think that's what we're looking at down the road.
He won't truly be testing free agency. He'll be testing 'restricted' free agency. I know, some team could try to outbid us, but they would have to willing to give up a 1st and 3rd to take him. I don't see it coming to that though. Once the new CBA is hammered out, he'll be signed long term within 24 hours.
If there was every a player to hope and pray they never get injured, I hope it's Harris. That guy has my utmost respect the way he's handled the 'business' side of things this offseason. Mangold is a close second...