I'm not bashing JE. He everything Welker was plus clutch. However the media and some fans have overplayed the impact at this point....
I think was JE provides for the team is probably as valuable, or at least close, to the team as what Gronk does. The difference being the fall off from Gronk to option two is about 25% productivity and I'm probably being kind. If DA was good to go it's probably about 75%.
I think you need to look at the system the Patriots want to run. The best offenses in the past nine years have been predicated on making the defense always wrong. Some of that is scheme, some of that is TFB, and a good chunk of that is the Patriots usually would have at least two players, and at the best three players who can break 1 on 1 man coverage in the first two steps.
Gronk is one of those players. The ideal 1 on 1 man to match him is 6-3, 245, runs a 4.5 or 4.6 forty and has at least safety coverage skills. Fortunately for the Patriots, there is one player like this in the league and he plays for the Patriots. Every other team has to devote additional resources to cover Gronk.
The Patriots rely on Edelman to beat his man so that he is either open, or the last remaining flexible resources on the defense are shifted to minimize the damage that he does. If he is open, then he gets the ball, if he is covered or shaded towards, the #3-5 options have room to operate against inferior defenders. When the Pats have the 3rd guy who demands attention (Lewis, Woodhood, Hernandez etc), the defense can never be right.
Gronk without Edelman will put up good numbers but he can be slowed down as teams can devote a linebacker to chunk, a corner to cover and a safety to bracket him.
Edelman without Gronk will put up decent numbers but he will not be a game changer as again the defense can contribute enough resources to contain him.
It's synergy.
