i'm sure they could've signed him, but only if they made him the highest paid WR in the league. (At what cost? As some have stated)
When you look at all the variables, as beane stated, and to put it in economic terms, the present value of expected benefits from the trade is greater than the PV from not trading. Benefits are not simply tangible like player stats and cap space, but also intangible like team-first mentality, etc. I think the one variable left out was impact on the fan base--good on Beane for that...