They didn't win very many playoff games with the Ben/Bell/Brown and never made a super bowl so they "pissed away their window" even when all those players were on the team. Over paying two players for not winning playoff games was not something the Steelers wanted to do.
I think they played the Bell situation just fine - couldn't agree on a long term contract, and you know the rest. The only other option was to pay Bell what he wanted and that is a bad precedent for franchises too - IMO. They did not play the AB situation optimally, but their hands were tied when AB used social media to grab all the leverage from the Steelers and (apparently) rejected a trade offer from Buffalo.
Paying players whatever they want is not a recipe for winning super bowls, you have to build a team. Now, I'm not saying the Steelers are going to win the SB this year or before Ben retires or whatever - but over paying Bell/Brown would not have gotten them there either.