Yeah Alex Smith is a game manager. So what? He's good at it. No he's not a 40 TD 4,500 yard passer, but he doesn't need to be, and won't get the opportunity with Frank Gore behind him. That said his career rating is only 78.8, but recently, i.e the last 4 years he has got better ever year, with year QB ratings going from 81.5 to 82.1 to 90.7 and this year 102.1! Now that he's finally on a decent team he has been doing great. Yes part of it is having Davis to throw to (as his most productive years line up with those numbers for Smith), but he has been managing games very effectively. Eli Manning has relied on his defense, so has Roethlisberger, so has Brady earlier on his career. They're still great QBs.
Welker wasn't decent in Miami. He never got a touch. He was an undiscovered talent that blossomed when actually given time (see: Drew Brees).
Branch hasn't been the same since he left New England. See
here for how similar his Seahawks and second stint Pats stats are.
Moss got his act together. That's it. He was widely reported to be a slacker after his first few amazing seasons in Minnesota. Bouncing around teams, BB actually got him motivated. Now he's old.
Tight Ends almost always have one on one match-ups, not just Gronk. That's the nature of the position. Actually watching the Pats will show you how often Gronk gets doubled, but it still doesn't matter.
Troy Brown - 1997 he recorded 41 catches for 607 yards and 6 touchdowns despite being behind both Terry Glenn and Shawn Jefferson" - quoted instead of rewording. That's a quality season. His solid numbers evident before Brady. He made one hell of a lot of big plays too.
I'll give you Galloway, didn't think about his age.
Patten - yeah, like I said great QBs help you out, but they're not everything.
Chad Jackson was a first round pick with great talent. Despite having Tom Brady throwing him the Ball he still didn't produce. Lacked the effort and the heart of a true great though. He bounced around the league to bad QBs and still did no better, so I think it's fair to say that the QB wasn't very relevant in the success of this receiver.