Edelman can probably play it, but he showed he has use on the outside
Setting aside the most obvious choice of talent level/experience etc, my two biggest concerns with trying to replace Welker with Edelman would be the lost chemistry with Brady/Welker, and the fact that Edelman has not really proven himself to be very durable so far.
I will agree with the notion that "Edelman can probably play it," although there would certainly be some downside to that along with lost production, just due to the talent level alone. When you add in the injury concerns to a position that Belichick himself admitted that "Welker gets blown up pretty good several times a game," (paraphrasing) there becomes a big question as to
how effectively Edelman would be able to play it.
I do agree with you, I think that Edelman has some niches that he fills nicely, and is still being developed and worked on. I think that he does some things well, and could even continue to improve more on the outside. It would seem that they recognize something there, and were attempting to continue to bring it out. Of course there is the ST effectiveness, the trick plays, and the more shallow routes that he and Brady had been working on together since last offseason.
I think Edelman has a role here, and I think he stays, but I also don't want to fall into the trap that he would be staying to "replace" Welker or his production, just due to that being nearly impossible and the fact that Belichick seems to like some of the other qualities about him as a football player.
The greatest thing about Welker to me is his toughness, he has to be one of the toughest SOB's that has played in awhile...On top of that, Brady really likes having him here and appreciates the fact that he knows where he's going to be pretty much all the time. And of course he continues to produce at a top level.
I'm all for giving Welker a nice contract within reason. My initial suggestion was 2/20 just due to the fact that I was leery of the front office's hints that they may not want to give him too long term of a deal. If they are good with 3/30, then all the better. He deserves it and should be retained as the #1 priority--clearly.