Let me temper the title a little - this isn't intended as a rigorous academic treatment of the Welker topic but rather me writing down a few frustrating thoughts I've had while reading others' Welker projections. I don't think I've read a single serious piece that projects Welker to improve upon his previous years... so I'll be the devil's advocate:
Section A: Fallacies
#1 Welker wont get as many targets!
In a healthy season P. Manning has averaged about 15 more attempts than Brady. In fact the last year the two were both healthy before 2012 (2010), Manning threw 187 more passes than Brady did. Also, Welker's reception numbers do not correlate with Brady's number of attempts, in fact in Welker's biggest year (123 receptions) Brady threw 20 fewer passes than Manning did last year for the Broncos (while lining up across from Randy Moss).
Conclusion: You can't project Welker's receptions or targets to decrease due to switching from New England to Denver by simply dividing up least year's passing pie.
#2 Welker is *just* a product of Brady and/or the Bellicheater System...
Usually this argument depends on comparing numbers between his careers in Miami and New England. Welker was not featured in Miami - he technically only "started" 3 games at WR in his 3 year career and was used a kick returner - so the comparison holds little/no weight. What happened when Brady went down with an injury in 2008? Welker caught 111 passes without Brady, whereas he averaged 112 when Brady was the quarterback.
Conclusion: Welker appears to be the best slot receiver in the game (and possibly in history) whether Tom Brady is throwing to him or not.
#3 But DT and Decker already own 50% of the targets - so Welker's ceiling is low
This is primarily refuted by #1 above (you can't just project targets using simple math and the previous years numbers). In fact, Wes Welker averaged more targets and more receptions when playing across from a star receiver (Randy Moss) than without him. Figuring out who will receive targets is more complicated than just ranking receivers and dividing the passing pie.
Section B: Real Differences
#1 Welker will draw the easiest coverage of his career
Since the exit of Randy Moss, Welker has often drawn coverage from top opposing cornerbacks. Due to the reputation of DT and Decker after last season Welker may be treated as an afterthought by opposing defensive coordinators purely out of necessity.
#2 Welker will lineup every day in practice against the best slot corner in the league
If OTAs are a sign of what is to come then Welker (and Harris) will get the best practice imaginable at their respective positions. For each, their Sunday competition will be the easiest opponent of the week.
#3 Risk versus Reward
Another Welker projection (I'm not saying who ;) mentioned Manning not using Welker as a "safety blanket like Brady did". Manning will absolutely use Welker as a safety blanket! That was probably a major motivation for Manning's support for signing Welker. Faced with a risky throw downfield and an open Welker near the first down marker, I suspect Manning will opt for the latter. Manning didn't have this option nearly as often last year - and when he did, he often took advantage of it.
last thoughts...: Point #3 above gets at another interesting logical fallacy. Most writters assume Welker wont cut into DT's production because they are different types of receivers. The quarterback does not have a set budget for deep and short passes. Any type of receiver can become the best option on a given play and the quarterback may choose to throw to the slot rather than go deep when the slot gets open. I expect that Manning will throw far fewer "risky" passes this year.