There are a couple of factors at play here. First, Johnson's statistics have never matched his reputation. Second, players of Johnson's ilk--above-average starting wings--have tended to decline in a hurry in their early 30s. By year three, just two of Johnson's top 10 comparables (Steve Smith and Jalen Rose) were offering their team any kind of value. A max deal for Johnson could end up very ugly.
As Hoopinion points out, this projection of 2010 free agents has 15 players with a higher cumulative Wins-over-replacement over the next three years higher than Joe Johnson, including Tyrus Thomas (and Thomas has a higher mark than every PF outside of Bosh).
That's in the first three years, not the next three that I think everyone accepts could get real ugly.
Note that Manu seems still very underrated (though saying it doesn't account for injury sort of nixes the whole idea in projecting the guy at this point), and David Lee is comparable to Amare and superior to Boozer.
What it also hammers home is that beyond LeBron/Wade/Bosh it's not that getting another guy would be consolation as much as combining two of them would be consolation.