I don't think this signals the Steelers believe the window to only be open for another year or two. With the way the cap is increasing the Steelers are absolutely fine, if they draft well on the defensive side of the ball. Neither the Brown nor Bell extensions will trigger cap related casualties of productive players in 2017 and it is doubtful that will be necessary for 2018 either. Brown's cap number didn't change at all and even if Bell's stays this high, they still have about $14-19M to sign their draft class, bring back Harrison and Timmons, if they want, and ink any other extensions for guys like Tuitt and Villanueva.
As it stands, the Steelers are actually in very good position to be in really good cap shape the next 2 years and have many options moving beyond that. As it always does, it will be determined by continuing to have a franchise QB and how they draft. If Ben gets seriously injured or they whiff on a draft class, the window will slam shut. Currently, it looks like they will have around $45M next year in cap room (before a likely restructure with Ben) to fit in Bell, a Tuitt extension, a Villanueva extension, and supplement the defense. That would include returning the entire 2016 offense, save DeAngelo Williams and backup OL, all DL starters, Shazier, Dupree, Williams, Mitchell, Gay, Burns, Golson, Golden, and Davis on defense. And, they could save about $5M by cutting Mitchell.
We all know the window closes when Ben retires because that is just how the NFL works, unless you get very lucky, but these moves don't signal them giving it just a year or two more IMO.