First, they already were limited in salary cap space, and will need $4 million to $5 million to sign their draft class. That means a veteran has to go or the club will have to convert more salaries signing bonuses. And that's no than using a credit card when you don't have any money.
Second, the one-year deal means Sanders is a free agent after the upcoming season. He really isn't good enough to warrant a franchise tag so he's probably gone -- maybe to the Patriots, who would not have to give up any draft compensation next year, as they offered to get Sanders in the first place.
Third, the Steelers were looking at a compensatory third-round pick for Stewart. In this draft that surely could have netted them a wide receiver like Steadman Bailey (West Virginia), Da'Rick Rogers (Tennessee Tech) or Marquise Goodwin (Texas), who figure to still be on the board in that range.
A draft pick would have been with the Steelers for three years at rookie wages.
Fourth, The Steelers could have packaged their third-round pick (No. 79) and the Patriots' pick (No. 91) and moved into the second round for a guy like Robert Woods or maybe Justin Hunter.
The Steelers missed out on some long-range possibilities by not taking the Patriots' offer for Sanders.