There is plenty of reps to go around with the way this team is constituted. Welker is likely headed for PUP and Randy Moss is given days off fairly regularly. Patten isn't going to need much time with Brady, and if - for instance - we obtain Deion Branch somehow, he - like the other vets - will probably be a part-time practice participant. Ultimately, I cannot think of an example where Belichick has jeopardized the team's ability to win in the short term for the sake of developing unknown, young talent at a position. He always brings contingencies - at least to camp. If the veteran contingency gets beat out - so be it, but I'd be surprised if he goes into camp counting on Brandon Tate, anything Tate gives you is a bonus.
I'm not against bringing in receiver competition. Quite the contrary. Bottom line, if Belichick thinks Tate has the potential to earn the spot opposite Moss in 2010, then it makes little sense to bring in Boldin/comparable because you pretty much ensure that Tate won't reach his potential. Poor use of resources (you want everyone to outplay their contract/draft position) and if you do it enough, your locker room starts filling up with youngsters wishing they were somewhere else.
You've run through the best case and worst case scenario pretty thoroughly - so I won't rehash, though I will say your worst case does not account for Randy Moss getting injured - which he did this year, but the lack of depth at the position forced him to play through it.
Essentially, I'd say the worst case scenario could be a lot worse. Just look at this season. Between Galloway & Lewis, we thought we'd have a WR depth chart as deep as 2007. Those guys were complete busts and we were forced to play a 7th round QB-turned-WR and a career STer - and that pretty much unraveled our season, just as much as any defensive struggles or AD shooting off his mouth.
It was less of a "worst case scenario" and more of the lower end of reasonable expectations. Moss is old, Welker is injured, Tate is new, Edelman has uncertain upside, Aiken is limited. You could make a case that they all need contingencies but unless you flush them all and start over, you really can't cover everything. For example, getting Boldin would be fine but you are right back where you started if Welker doesn't come back and Tate is a flop. Add in a Moss injury and Aiken is starting again.
And the upside of aggressively addressing the wide receiver position is just too high. The last time Brady was given ample weapons, they blew away offensive records and damn near went undefeated. In 2003/2004, when he had what I'd say is clearly the 2nd best unit he's dealt with - they won Super Bowls.
2003/2004 had Brown, Branch, Patten, Givens and Bethel. Patten was injured in 2003 and Bethel's primary contributions were on ST. In this configuration, Edelman has to fill the Branch role and Tate has to be Patten. If you don't believe that either of these guys can reach that bar, then I understand why you want someone else. BTW to finish out the comparison, I'm looking for the Pats to draft Seyi Ajirotutu to play the Givens role. Also, I didn't mention Welker and if he returns for the stretch run (a reasonable possibility), the depth chart of Moss, Welker, Tate, Edelman, Ajirotutu/comparable, Aiken compares pretty favorably to 2003/2004.
IMO, chasing 2007 is both futile and foolish. Getting the consistency and clutch play of 2003/2004 is a worthwhile goal. I thought drafting Tate and Edelman was a step in that direction.
If they had just one more capable receiver in 2006, I have no doubt we probably have one more ring. If they had one more capable receiver in 2009, I'm damn sure we make it past the first round. The reward is too high not to add wide receivers, whether it stifles Tate's development or not. If Brady has 3 or 4 legitimate options, we score points against any defense.
They had one more receiver in both 2006 and 2009, but they didn't turn out to be capable (CJack = underachiever, Tate = hurt). The player acquisition effort was there, but the results weren't. In the end, Belichick has to make the call and it didn't work out in 2006 and 2009 (he would almost certainly admit that as well). We will see this offseason whether Belichick is hedging his bets on Tate and Edelman.